Early in the movie, the General says that the launch codes wont work unless they are at defcon 1. When they decide not to Nuke 'em at the end, why didn't he just change the defcon to 5 so that the computer wouldn't be able to launch the missles?
They simply couldn't - WOPR was partially responsible for establishing current Defcon level, so the humans really fell off the loop of control.
The back door password was "Joshua" because the guy who made the computer had a son named Joshua. Because of this, David often refers to the computer as Joshua, even after he discovers that its real name is WOPR. Later in the story, Professor Falken refers to the computer as Joshua. Why would he refer to his computer by his dead son's name?
Turning into Fridge Brilliance: It's Rule of Symbolism. It's almost like Frankenstein. The "father" abandons his "son", who's turned to darker purposes, only here, once his "father" acknowledges him for the first time in many years, the son returns from teetering on the edge of destruction.
It also underscores that the rest of the WOPR staff has little insight into just how advanced Falken's creation is. They all refer to it as WOPR, after the computer's physical hardware, but David and Falken refer to Joshua directly. In context, it suggests that the military didn't know that the simulator was self-aware, much less that it needed safeguards to prevent it from commandeering the American nuclear arsenal.
Joshua supposedly doesn't understand futility, but is programmed to play the (unlisted) Tic-Tac-Toe game, and more famously Chess. While it's probable the former game is legacy code that Falken never bothered to play with Joshua, chess is rife with stalemates. We have no direct indication that Falken ever intentionally tried for a stalemate, so either Joshua outsmarted him every time Falken tried, or Falken never attempted it.
The difference being chess can end in a stalemate; the goal was to make Joshua understand some games always do.
Chess also relies on acceptable casualties, as it's almost impossible to reach checkmate without losing a few pieces along the way. If anything, chess would impart the opposite lesson of what Falken was trying to teach Joshua - that it's totally okay to lose a lot of pieces along the way as long as you take the king.
Why Global thermonuclear war has only USA and USSR as playable nations when there are other nations with nukes? Because WOPR only has data for USA abd USSR
Even though acoustic modems were outdated by 1983, modems in general still weren't cheap. An acoustic, likely scavenged from a local university, might have been all David could manage to get ahold of for his obviously-cobbled-together rig.
Word of God deliberately gave David equipment which was outdated even then, so as to explain how he had these things. They said that it was Truth in Television: as somebody no longer needs some stuff, he'll give them away, and a kid like David would gladly accept it.
Given David's ties to the local software geeks, it's probable that he inherited it from them.
Joshua is an AI, but only understands how to play games and use its directly available resources, like the phone directory to find David. This is why he activates the nuclear arsenal when David begins playing Global Thermonuclear War — the commands for the simulation were identical to reality — but also why Joshua assumed David was Falken for calling Joshua by name, even when Falken preferred chess over other games and was accessing Joshua from Seattle instead of Falken's home or the WOPR base. We later see that the most advanced technology in the world is physically incapable of sustaining him at his full potential. Due to his limited interaction and the inadequacy of his hardware, Joshua is essentially just a clever child.
The two soldiers at the beginning of the movie apparently didn't know they were in a simulation. When one of them wouldn't turn his key, the other threatened him at gunpoint (apparently according to protocol) to follow orders. The follow-up discussion implies the first soldier continued to refuse. We're not told if the simulated nature of their situation was revealed before the second soldier could follow through on his threat.
Both soldiers were shown in a later scene showing the operators' seats (and other relevant human equipment) being removed in favor of the new WOPR automated parts.
This was a training exercise, even if the two didn't know it at the time. They were probably being monitored by superiors who would turn off all the flashing lights and announce over a loudspeaker that it was a drill. A drill which could potentially (according to the regulations here, obviously) result in pulling a weapon on your partner, the weapons are probably issued and loaded with blanks, or more likely completely dummy ammunition.
Actually, they are Airmen, not soldiers.
There were several non-nuclear war scenarios listed as "games" that David could have selected, including chemical and biological weapon attacks. Presumably, WOPR has a say in coordinating those for the military also, else they probably wouldn't have been listed. If David had decided one of these options looked more interesting than simply nuking the hell out of everything, could WOPR have set off a Real Life military conflict, by faking reports of the use of such weapons? It's a lot harder to verify whether gas or germ attacks have been used at ground-level than whether or not a city has vanished in a mushroom cloud, so if WOPR- had been playing such a "game", American troops might've actually been dispatched to investigate such a fake report, and the place where the "attack" allegedly happened could have interpreted their arrival as blatant U.S. aggression. Granted, it wouldn't have risked the whole planet, but people might have really died in such circumstances ... and WOPR possibly could find a winning scenario for such a "game", so might never have stopped "playing" it.
Nevermind the kinds of biotoxic and chemical weapons that WOPR might have at its disposal, and might launch in retaliation.
American doctrine (at that time and still currently, as far as we know) was that a strategic attack with chemical or biological weapons would be answered with nuclear weapons. So yes, if WOPR could convince NORAD that such an attack had occurred, nukes would have flown. However, it would have been far more difficult because unlike with nuclear attack (where you're going to launch as soon as you've confirmed that missiles are headed towards you, before they even land), in this situation you're not going to nuke Russia until after the "gas" or "germs" have been released upon a city — and of course since no actual attack is taking place, the illusory nature of the attack will be revealed the instant anyone speaks to the target area on the phone.
How bad would things have gotten had David decided to play as the Americans instead of the Soviets?
Nothing would have happened at all. The Soviets can't see anything WOPR is putting on the screen at NORAD, because nothing is actually happening in the real world and they obviously have no access to NORAD systems. And while there's going to be some heart attacks in NORAD at seeing their own missile systems apparently all go to launch, the panic will be momentary and stop as soon as General Beringer calls the silos to ask them what the hell they think they're doing, and they answer back "... ummm, doing what, sir? Nothing's happening today."
Unless WOPR set itself at Defcon 1 and decided to launch anyway, as occurred at the ending of the film. What would have happened if this occurred before anyone realized what was actually happening?
Likely, World War III would be the end result. If you consider Joshua was willing to "take it outside" as it were, it's very likely the nukes would have flown before anyone could have stopped it. All it would have taken was David say, "Nuke Moscow" and, likely, Joshua would have done just that. Russia would have immediately retaliated the moment they saw the nukes heading their way. Even if Joshua didn't send the nukes, it's highly possible, judging from the tensions of the Cold War, that Russia, if they received an incoming bogie report, would have retaliated in defense, even if just to shoot down the incoming missiles. If the anti-nuke missiles failed to blow-up mid-air, or even if they did blow up mid-air, wherever those missiles landed and exploded would request the UN's or at least the US's aid in retaliation. If Russia claimed they received a report of an incoming nuke, likely nobody would believe them unless the US admitted their system sent that fake notice. If the US admitted to Joshua doing that, Russia would likely take action for the US's act of instigation. The US would be left with no choice but self-defense at that point. The UN would either be split in whose side to take, take a neutral position until the fighting stretched into another nation's territory, or would be a 3rd party trying to end the war before nuclear weapons could be used... honestly, I see options 1 and 3 as the most likely. It's not that a neutral position would have been impossible, but it would have been too risky especially after considering Nazi Germany got so powerful because surrounding nations refused to step in and stop Germany's conquest. Additionally, there is precedent for my assertion based on how the major political powers (even disregarding the US and USSR) were involved to some degree with the Vietnam and Korean Wars.
The doctor's recorded notes state that his patient's condition was consistent with the use of marijuana and/or PCP. Inside NORAD. Give that a second to sink in.