The ending takes on a completely new meaning with this film as the backstory. That film's ending originally focused on Bruce and Lois' reaction to Clark's death, and Diana agreeing to help Bruce assemble the League after spending almost a century retired from superheroics comes off as forced. However, this film implies that Clark's sacrifice reminded her of Steve's, and inspired her to come out of retirement, as a way of honoring Superman (and through him, Steve). Her comments at Clark's funeral seem to cement this. "They don't know how to honor him, except as a soldier." Is she talking about Clark or Steve (who doesn't appear to have gotten a funeral)?
It also changes her motivation for wanting the photograph so badly. She didn't want it so she could hide her identity; she just wanted the only physical reminder there was to a defining moment in her life. When you think about it, Lex must have had the original or known where to find it in order to have a scan of it in his files. Bruce would be able to use that information to find the original's location.
Diana full-on Squees when she sees a baby because she's never seen one before, being the youngest Amazon and the only child ever born and grown up on Themyscira.
Sir Patrick pausing and staring at Diana in the middle of his speech is played as if he's simply stunned that a beautiful woman has invaded a government council, but it's revealed that he's actually her brother, Ares, so his reaction is one of recognition.
Knowledge of Diana:
One would wonder how nobody would know about a superpowered woman who single-handedly helped the British liberate a German-occupied town in WWI, as out of the thousands of British troops and villagers that witnessed her feats, at least some of them must have talked. However, in the film, it's shown that Ludendorff gassed the entire town, killing the villagers and presumably all of the soldiers that took part in the battle. This would leave only Samir, Charlie, Chief, Dr. Poison, and a handful of German soldiers as the only witnesses to Diana's existence.
It appeared that most of the soldiers stopped at the trenches, and only Diana and her friends kept moving forward into the town. However, even if the soldiers talked about a mysterious armored woman who led the charge across no-man's-land, swatting bullets out of the air and leaping into the trenches to destroy the machine-gun nests, the stories would probably be dismissed as mass hysteria or shell shock.
Given how volatile Dr. Poison's chemicals were and Germany wanting an end to the war, on their side it may have been written off as sabotage and a failure on Dr. Poison's part.
DC-verse historians probably count those accounts as an offshoot of the Angels of Mons stories.
When you think about it, General Ludendorff's ego was his undoing, along with One Dialogue, Two Conversations. Typical of the German regime, General Ludendorff believed himself invincible, even god-like. By unknowingly feeding Diana's belief that he was Ares with his talk of being one of the Gods, Ludendorff brought about his own downfall.
In the prologue, Ares' sword looks identical to the "God-Killer" sword, suggesting that the God-Killer sword is his sword. This would explain why the Amazons mistake it for the God-Killer and why Ares destroys it so easily when it's turned on him.
From Batman v. Superman to Wonder Woman, there are 3 scenes total with Diana at fancy dress galas. In BVS, she wore a red dress at the first gala and then a white one at the second. At Ludendorff's gala, Diana appropriated the blue dress. Diana's ball gowns are each in a color from Wonder Woman's iconic costume. Only the gold appears to be absent, though her white dress had a large gilt clasp at the neck, so all her costume colors have been included.
Sir Patrick secretly gives Captain Trevor the money he needs to at least start his mission to find the new secret weapon, despite officially being against any action that would risk the Armistice. Of course, Sir Patrick is actually Ares, doing his usual thing and giving the mortals ideas and tools to kill each other with. The fact that it's not enough money to pay the whole way leaves Steve and his men the option of giving up and going back, or committing to follow through of their own free will. It's also a Xanatos Gambit. If they manage to stop Ludendorff and Dr. Poison, he gets the armistice that leads to World War II and he just has to wait a little while. If they don't, his "armistice" deal is off the table and he gets a world-killing war anyway.
Actually, it could be a lot simpler than that. Sir Patrick is just getting Diana as far away from himself as possible so she doesn't screw up the peace talks. If Diana realized that he was Ares when she first saw him, and attacked in the middle of a room full of allied generals, all hell would've broken loose. Now, Ares had disguises, but if she stuck around and found him out in an even more inconvenient location (like, say, while they were brokering the armistice deal?) and attacked him, it could've been worse. He was preserving the armistice by moving Diana to the one (okay, one of several) place(s) in Europe where attacking people and killing them on suspicions of treason would be the most accepted.
Also, by funnelling Diana to no man's land, he's trying to force Diana to see the worst of humanity and convert her to his way of thinking. Unfortunately, it backfires, since suffering with them only makes her empathize with humans more.
Why do the German high command shoot down Ludendorff's demand of "just one final attack" to destroy the Allies? Throughout WW1, the stalemate of trench warfare led many generals to dream of a 'big push' or massive assault that would throw back their opponent's defenses and allow their troops to finally gain ground. However, these often ended in disaster, such as Verdun and the Somme offensives, which saw almost an entire generation of men wiped out. As military tactics adopted to modern weaponry, a 'big push' was attempted by Germany in Spring 1918 but petered out due to shortage of human reserves and food, and finally achieved by the Allies, when the Hindenburg Line was broken in September 1918, the last major line of German defense. By November 1918, when the movie takes place, the German high command know that not only is their defense gone, but they've seen the aftermath of various 'big pushes' too many times and that Ludendorff's dream is hopeless.
And who was it who devised the Spring Offensive earlier that year? Ludendorff. The very same Ludendorff now demanding one more attack.
Furthermore, Ludendorff was also responsible for the food and troop shortages by redirecting troops east to occupy land ceded by Russia, and doing nothing to mitigate the food shortages.
And worst of all, it was Ludendorff who spearheaded Unrestricted Submarine Warfare, which was one of the reasons that ultimately brought America and its massive industrial base into WWI. With all this taken into account, the question is not "why wouldn't they trust him", the question is "why would they trust him?"
Geoff Johns was executive producer of the film, and there appeared to be a stealth reference to Blackest Night. In that arc, Wonder Woman was deputized as a Star Sapphire (Green Lantern-type rings powered by love). What is the ring's incantation when choosing a wielder? "You have known great love and experienced great loss. We can give you purpose." The climactic moment? Well, Steve counts as "great love and great loss." (So does the death of Antiope and leaving Thymescira and her mother forever.) And that love gives her the focus to both show Dr. Poison mercy... and show Ares none.
The German high command refer to Doctor Maru as "Ludendorff's witch". This description proves more accurate than they would ever realize, as the worst of her poisons, potions and other concoctions prove to be in large measure inspired and enabled by supernatural means. Not to mention her grotesque true appearance...
After Diana kills Ludendorff, she finds Sir Patrick inside the building and he reveals that he, not the German official, is Ares. Diana reaches for her sword, the "God-killer", only to realize that it is still on the rooftop lodged inside Ludendorff. The movie cuts to what Steve and his buddies are doing, implying that she would be forced to fight Ares without it, but when the film cuts back to her, she's confronting him with sword in hand. The fact that he allowed her to retrieve it proves that he doesn't fear the weapon and that it can't actually kill him. Ares might have wanted her to fight him with the sword so he could deliver The Reveal to her and make his case for joining him.
Why did Hippolyta look worried when she caught young Diana by the wrist after she deliberately jumped off a ledge? It's not because she was worried Diana would hurt herself, it's because she was worried young Diana would discover that it wouldn't hurt her; she'd discover she's more than just the "only child amongst all the Amazons". Compare the moment to Hippolyta's later worries about Diana discovering her true nature.
Diana working as a museum curator makes sense when you consider that she decided to study human history and culture after seeing and realizing that they will always fight each other. If they don't deserve to be protected, at least their cultural heritage does.
As Antiope is dying, she says to Diana, "You must... God-killer... Diana, go." Upon first viewing, both Diana and the audience assume that she's telling Diana to wield the God-killer sword. After The Reveal, you realize she was actually trying to tell Diana the truth about her heritage, and is in fact addressing Diana as "God-killer."
The god of war arranging a peace armistice sounds odd, but Ares is a long-term manipulator and strategist. One long ongoing war would eventually wear everyone out and theyd just call it quits and not do it again. An interval of peace would give Ares time to come up with new weapons and new ways to make people turn on each other and start war again.
With the Word of God reveal that Chief really IS Napi, a Native American trickster god, his introduction to Diana has a bit of foreshadowing to it. Like Ares earlier in the film, Chief realized almost immediately that Diana was a divine being like himself. So he decided to properly introduce himself in such a way that she would be able to understand him... but keep his secret from the rest of the team.
Through the first act of the movie, multiple characters try to shake Diana's hand, only to be refused. This makes sense, because she's unfamiliar with the customs of the modern world. But then she shakes Chief's hand, raising the question: Why did she refuse everyone before? Because Chief was the only person worthy of her respect she'd met so far.
In one shot, just before the part where Diana climbs the armoury wall, we very clearly see a male cattle with antlers. This establishes that there are male animals on Themyscira, and it's only the humans who are entirely made up of women which in turn helps explain why Diana immediately recognised Steve as a man.
When he tells his story to the Amazons, Steve says he doesn't know why he decided to steal the book, just that a feeling came over him. If he doesn't grab the book, then Dr. Poison finishes her work sooner, and the Germans can outright win the war. But if he brings it back successfully, there's a chance that someone on the side of the Allies could argue for creating the gas to use it against the Germans, thereby exacerbating the war and its casualties. So why did Steve suddenly buck his orders to steal the book? Because of Ares.
Why did The German soldiers give up after Ares was beaten? Think about it, their commanding officer Ludendorff is dead, the next person up in the chain Dr. Poison has fled, their gas bearing planes have been destroyed and they just witnessed a death brawl between what could only be described as two gods that ended with the woman who killed Ludendorff on top. They're not magically giving up because Ares is dead, they're giving up because they Know When to Fold 'Em.
This is Truth in Television and Reality Is Unrealistic. As shown in documentaries like They Shall Not Grow Old, German trench soldiers who were captured often stopped fighting immediately, and even took to carrying wounded British, etc., soldiers back with them to the enemy camps on stretchers, as well as comfortably joking and swapping hats with the trench soldiers who had taken them captive.
And there's another level to it: When the final battle is over, the Germans are shown hugging/comforting one another, relieved that it's all over; and one soldier can be seen kneeling beside a fallen comrade, clearly mourning. It's meant to show the Not So Different aspect of the opposing sides which, as has been pointed out elsewhere, is why it was such a masterstroke to set Diana's debut into Man's World against the somewhat more morally ambiguous WWI rather than the very polarized WWII as she was originally.
Sameer can keep up with Diana in Chinese and Spanish, but is baffled by her knowledge of Ancient Greek. This makes perfect sense-as a spy/infiltrator, he'd naturally know all kinds of modern languages so he could fit in there, but there would be no reason for him to learn Ancient Greek!
While it's still present, the feminist themes of the movie are significantly downplayed, despite its main character being the feminist icon for almost a century now. This makes sense, as while Diana is female, she is also a god, which separates her even from other women, as seen by how the Amazons treat her when they see the true extent of her powers at the beginning of the movie. Ultimately, the story of the film is not about a woman trying to understand (and change) man's world, but rather a god trying to understand humanity's world. This is further supported by how Rule of Symbolism uses Dr. Poison — a woman — to represent humanity's faults.
Doctor Maru had more of an influence on Diana's disillusionment with humanity at large than one may think at first. Being from a society of warrior women who have lived for centuries under a strict code of honor, she more than likely holds females to a higher standard of behavior than males. Note how disgusted Diana is with the British high command who have no shame in being literal Armchair Military who callously expend their soldiers' lives, and how she (at least initially) regards Charlie a coward for his sniper tactics. "Doctor Poison" is a Mad Scientist who devised (with help) a horrific weapon that kills hundreds from great distances. She is unabashedly evil and sadistic, the polar opposite of the likes of Hippolyta and Antiope, the worst that humanity has to offer, and she's a woman. Despite her overall egalitarian mindset, towards the end of the film, Diana had to be thinking "if even a female can be so degenerated and corrupted by this world, what hope is there for anyone?"
The chemical weapons developed under Ares' influence were bad enough, but try to imagine if he had managed to live long enough to influence World War II and see the nuclear development... This also raises an even more disturbing thought. Ares has absolutely no involvement at all in World War II. He might have influenced Dr. Poison to come up with the deadly gas, but it's humans who came up with nuclear weapons and made the decision to use them. Not to mention using gas as an efficient method for conducting genocide...
Nobody ever saw Wonder Woman again after World War I ended until the present day, not even during World War II or the Cold War's proxy conflicts. Whatever happened after WW1 caused Diana to drop off the radar completely and not showing up to help humans like she had on her first outing. It's highly likely that she has taken many levels in cynic after what she had gone through during her fight with Ares and the loss of Steve. Additionally, the atrocities that happened before and during World War 2, such as the bombing of Guernica, the Nanking massacre, the Holocaust, or the atomic bombings of Hiroshima and Nagasaki could have enhanced this effect and decreased Diana's confidence in humanity.
Patty Jenkins and Gal Gadot however have confirmed that Diana didn't turn her back on humanity as implied by Batman v Superman. However, given how WWII and the Cold War still happened in the DCEU timeline, this would still be terrible. Even if she did intervene, millions would've still died anyways and she would've failed to end humanity's penchant for war.
Doctor Maru was spared at the end, possibly because she was completely defeated. But there is nothing to say that, after getting away, her "talents" couldn't be purchased by the next power-mad lunatic to come along. Assuming that she wasn't just given a field execution for war crimes by one of Steve's mercenary pals off camera and out of Diana's sight.
General Erich Ludendorff was a real figure during World War I, and while he wasn't exactly a nice man, he's nowhere as evil as he is portrayed in the movie. That's because in real life, he never met Doctor Maru, who provides him with deadly weapons, and Ares doesn't exist to fuel his lust for war. While Ares claimed that he technically wasn't responsible for humans going to war, his indirect involvement by providing Maru's deadly weapon ideas certainly influence Ludendorff to embrace his war-mongering ideology. It could even possibly be Ares himself who directs Maru to Ludendorff in the first place. Ares's mere presence alters our history as we know it. Why Ludendorff's large mustache is gone is another matter entirely.
Diana was the only child who grew up on the island, which means since the Amazons have no offspring, their numbers can only get smaller. The Amazons couldn't possibly last as a fighting force in the outside world.
Although we don't know what happens when an Amazon has a child with a mortal man. Does the child end up being a full Amazon, a full human, or something in between? If the former, then their only hope of long-term relevance would literally be the Power of Love.
The ending seemingly has Diana triumphing with the power of her principles, destroying the god of war and causing the German soldiers to lay down their arms in relief as peace is finally brought to the world. And then you remember that Diana has lived to the present day, and had to witness a second global conflict with greater acts of evil and a higher death toll than the first.
A bit of a blink and you'll miss it moment, but the mother and baby that Diana encounters in the streets of London: Her clothes are shabby and filthy, and she's not passing through the street but sitting on the side of the street. She and her baby are likely homeless, quite possibly the widow and orphan of one of the countless soldiers lost in the War. As this was during the days before war widows were given pensions, everyone else on the street will likely regard the woman and her child at worst as pests to be rid of, at best as people to be pitied, but not helped. Only Diana sees the baby as a miracle, and she has no concept of homelessness, or widowhood.
World War I is sometimes derisively referred to as a big, destructive family squabble due to the fact so many of the countries' crowned (and sometimes elected) leaders were cousins of one another. Diana having to tangle with her half-brother would certainly fit the theme of "stupid inter-family argument that drags a bunch of innocent people down with them."