World War 2: Pacific Theatre: Grave of the Fireflies
Our only animated movie, but we really cannot avoid it. It is the story of two siblings caught in the American firebombing campaign over Japan. It's a touching tale that is in my opinion Takahata's masterpiece (he co-founded Studio Ghibli with the more known Miyazaki). The animation is of great quality as well.
The Napoleonic Wars: Waterloo
Despite being about Napoleon's defeat, this is my favourite movie on the Napoleonic wars simply because of the cinematography. You might not know, but to make that movie, Bondarchuk rented a parcel of land from Ukraine in order to actually recreate the battlefield. Then, they hired an entire Red Army Division to play as extras...as well as a cavalry brigade (2000~ horsemen). This is golden age film making at its best. It also helps that the cast is rock solid, even though there is plenty of ham, it works great.
Vietnam War: Apocalypse Now
Captain Willard has a mission: he must kill renegade Colonel Kurtz. But as Nietzshe once said, stare into the abyss, and the abyss stares back into you. Willard and Kurtz will both face their demons and come to understand the kind of mettle that the waging of this new kind of war requires. A great portrait of the fucked up psychology of Vietnam combat and the path we could have taken. It is also a stark reminder of the danger of losing our humanity. Honorable mention to Full Metal Jacket.
American Civil War: Gettysburg
And just for you /u/Striding_Huntsman, a Civil War movie. Gettysburg is pretty much the landmark battle for the entire war, the decisive engagement (even though in actuality it wasn't any single battle that defeated the South). Similarly, the movie is one of the better realized period pieces on the Civil War. Glory, while having an interesting topic, is too narrow in scope (which is fine).
Well there you have it. Leave hate, love, suggestions, requests in the comments.