Situational Irony: The Age of Enlistees
Many people who enlisted to fight in WWI were only 13 or 14 years old, far too young to fight in any war and far too young to want to fight in any war.
Dramatic Irony: The War's Outcome
Audiences know before everyone pictured in the film that the Allies - led by Britain and France and the United States - beat the Central Powers in the war.
Situational Irony: Crossing No Man's Land
Everyone in each army voluntarily crosses No Man's Land - a surefire death sentence.
Dramatic Irony: The Fates of Soldiers
Audiences members know the fates of the soldiers featured in the film. The characters, whoever, do not.
Situational Irony: Willingness To Go To War
Most of the men featured in the film willingly go to war knowing full well that they will likely die.