The Battle of Algiers (1966)
Modern conflicts are characterised not by the sweeping charge into battle, but by the guerrilla war that takes place in the aftermath. The Battle of Algiers, then, is a true modern war film.
It's a remarkably even-handed depiction of the guerrilla insurrection that took place during the Algerian War of Independence.
Neither the Algerian revolutionaries nor the French counter-insurgency come out of it particularly well, with both engaging in atrocities as the conflict drags on.