Having solved Elfrieden's food crisis and building up the country's economy, Souma's attention turns to dealing with the internal threat of the Three Dukes: Georg Carmine of the Army, Excel Walter of the Navy, and Castor Vargas of the Air Force. With the Demon Lord's Domain in the north and the nearby threat of invasion, especially from the Principality of Amidonia, Souma needs to unite the country's military for them to stand a chance. Souma eventually gives the Dukes an ultimatum: join him, or he will go to war against them. Souma gets Excel on his side (thanks to his connections with Juna, Excel's granddaughter), but the other two refuse to take his side, setting off the One Week War, which starts as a civil war in the kingdom.
Dealing with two of the Dukes provides the perfect opportunity for the Principality, which lost land in a war with Elfrieden decades ago, the chance to launch an invasion of the kingdom. Through a series of inventive battle strategies, Souma's army is able to take out the Air Force and Army within a couple of days, while stalling the Amidonian invasion. After bringing the entire military under his control, Souma launches a counter invasion, going after Van, the Amidonian capital.
This leads to a final battle for the city, with the Kingdom of Elfrieden overwhelming the Amidonian forces, eventually taking their capital. While the entire operation was a success, Souma just barely avoids getting killed by Gaius VIII, the Sovereign Prince of Amidonia. Souma also starts to see himself becoming the king more than being a person, a change that leaves him afraid of who he might become in the future.
Tropes
- Ain't Too Proud to Beg: Weist Garreau, Lord of Altomura, stalls the Amidonian army outside his city with this by pretending to surrender to Gaius and begging him to order his army to to stand down while he talks everyone inside into surrendering as well. Being a proud warrior who sees Weist giving up without a fight as pitiful, Gaius agrees, not realizing that he's just being stalled while Souma settles the Civil War.
- Aristocrats Are Evil: The nobles that seek refuge with Carmine. They opposed Souma replacing them with actually useful vassals and are only interested in protecting their own holdings, even going as far to steal weapons from the army that Carmine was holding off on using for a reason because they got desperate.
- Becoming the Mask: After spending the first volume talking about how he'll abdicate the throne to Liscia the first chance he gets, Souma finally starts to accept being the king. By the end of the volume, he thinks he's taking it too far, losing himself in his role.
- Big Badass Battle Sequence: Most of the book is jumping from one of these to another, from the battle outside Randel, to taking Red Dragon City, and finally to the battle outside Van.
- Big Damn Heroes:
- The dark elves who join the battle outside of Randel. Their surprise arrival gives the Forbidden Army enough soldiers to make fighting a defensive battle viable.
- The Air Force coming to the aid of the Royal Army
- Chekhov's Gunman: The father of the girl that Hal rescued at the landslide shows up with the dark elf soldiers that join the Forbidden Army's fight outside of Randel.
- Citadel City
- Randel is one, being the headquarters of the Elfrieden Army. All the battles actually take place outside of the city so that they don't have to try and storm the city.
- Van, the capital of Amidonia. Just like Randel, the battle takes place outside of the city for exactly that reason.
- Civil War: The first part of the One Week War, with Castor and Carmine leading their forces against Souma.
- The Confidant: Souma turns to Carla of all people to admit that he feels like a fake on the throne, having to play the role of being king instead of actually being one.
- Conscription: Amidonia does this to make up for the soldiers lost in Juna's sneak attack on their way to defend Van. While it bulks up their numbers, they fall short in the actual fighting.
- Curb-Stomp Battle: Souma manages to maneuver his way into a number of these, despite being at a disadvantage on paper in almost every fight.
- Castor and his personal troops end up being no match for a battleship on wheels distracting them from a small commando unit capturing Castor.
- Once the Air Force joins in, the battle at Randel ends almost immediately. Justified in that Carmine wasn't even trying to win, so the army never put up a real fight.
- The battle outside Van was overwhelmingly in Elfrieden's favor, with a much larger and well-rested army meeting on a wide open field. The only suspense of the battle came from Gaius and his troops using almost suicidal tactics at the very end.
- Decapitated Army: Invoked. Souma and the Dukes all agree that if any one of them is captured or killed, they will immediately surrender command of their troops to the victor. This allows Souma to quickly bring the entire military under his control and take on the Amidonian troops.
- Enemy Civil War/Let No Crisis Go to Waste: Amidonia uses the rebellion in Elfrieden as an opportunity to launch an invasion of their own.
- False Flag Operation: To clear out towns near the border with Amidonia, Souma pilots some dolls with fire on their heads, then puts out a quest with the Adventurers' Guild to help people evacuate the towns, claiming the dolls are a new type of monster attacking the kingdom. It works to get the people out and burn down some buildings that would otherwise have been useful for the Amidonian army.
- Fighting for a Homeland: While the Principality of Amidonia still exists, regaining their lost lands is the main motivation for them going to war against Elfrieden.
- Gambit Pileup: While there was some movement in the first volume, it starts in earnest in this volume, setting up a mess of schemes and gambits that lasts through the fourth volume.
- The Dukes start a civil war in Elfrieden against Souma, though the actual purpose for the rebellion is for Georg Carmine to draw out the corrupt nobles and take them out all at once.
- Amidonia uses the civil war in Elfrieden as an opportunity to invade Elfrieden.
- Excel starts working with Souma ahead of the ultimatum, and plays a large part in the siege of Altomura, which slows down the Amidonian invasion.
- Souma and Hakuya play Xanatos Speed Chess, trying to react to a constantly changing situation, especially when they never expected that Castor would side against them in the conflict.
- Graceful Loser: After Aisha and Liscia defeat Castor, he takes it in stride, only asking Liscia if Souma is a good king and asking Carla to surrender as well.
- Important Haircut: Liscia cuts her hair short during the ultimatum with the Dukes, as a way of showing Carmine her determination if they meet in the upcoming battle.
- Improbable Aiming Skills: The dark elves are so good with their bows that they can hit and destroy cannonballs while the latter are mid-flight.
- Inappropriately Close Comrades: While Kaede and Hal aren't explicitly said to be in a relationship yet, she probably shouldn't be his commanding officer.
- Interservice Rivalry: Blown up to full civil war levels. The Forbidden Army (the king's troops) end up fighting the country's Army and Air Force, with the Navy siding with the Forbidden Army. By the end of the volume, they've all been united and are directly controlled by Souma himself.
- Last Stand:
- Gaius VIII puts up one hell of a fight even after it's clear he'll never win, coming up just short of killing Souma (his sword stuck in the ground at Souma's feet) in the battle outside Van.
- Wanting to avert this is one of the reasons Garreau's gambit works - a pitched urban battle against Altomura's entire population would cost Amidonia far too much in soldiers and time, plus any survivors would hate Amidonia and potentially rebel the moment they saw an opening, thus requiring a garrison that would be another drain in resources.
- Loophole Abuse: Amidonia justifies their invasion of Elfrieden by claiming that, since Elfrieden never signed the Mankind Declaration, they can't be protected from invasion under its rules.
- Revenge Before Reason: Amidonia's approach to Elfrieden, seeking to get back their lost lands ahead of any other, saner goals.
- Ridiculously Fast Construction: Based on the legend of Sunomata Castle (a fort built during the Sengoku Period, according to said legend, in one night - although it was a couple of days in reality), Souma conceives a plan where the Forbidden Army actually builds an entire castle in one night.
- The Short War: The Civil War against Dukes Castor and Carmine and the following counter-invasion of Amidonia comes to be known in-universe as The One Week War, because that's pretty much how long everything took. Castor has captured after his Air Force was lured out of his city, and Carmine always intended to surrender in order to hand over the corrupt nobles who gathered under him to Souma, which meant that Prince Gaius and the Amidonian army are caught with their pants around their legs when Souma announces the end of the civil war and intent to use the reunited army to invade Amidonia. In his rush to return to the capital of Van, Gaius's army is weakened by both the forced march and guerilla warfare, and he ultimately meets his end when his crippled army is met by Elfrieden's much larger and better rested army a good distance away from the capital.
- The Siege:
- Invoked with the battle outside of Randel, where the Forbidden Army builds a fortress overnight outside of the city, making their battle defensive rather than offensive to make up for being outnumbered nearly 4 to 1.
- Averted by Weist Garreau, Lord of the city of Altomura. The Amidonian forces could easily defeat them in a siege, so Weist offers to hand the city over if they just give him time to convince everyone else. He ends up stalling long enough for Carmine's forces to surrender, preventing Elfrieden from needing to fight a war on multiple fronts.
- Superweapon Surprise: When Souma sends the Albert battleship (converted to run on wheels) to attack Kouryuu Castle, Carla leads the air force wyverns to attack the battleship - only to run into the Albert's anti-air defenses. Downplayed in that the weapon already existed - but no one expected a ship to have it installed, since the concept of aeronaval warfare is unknown.
- Warrior Prince: Gaius VIII and his son Julius, sovereign prince and crown prince of Amidonia, respectively, fight in the front lines in the battle outside of Van.
