In The Pity of War: Explaining World War I (1998), Niall Ferguson claims that "men may have fought  simply because fighting was fun." This is an exact quote (page 90).
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https://www.google.com/books/edition/The_Pity_of_War/1Pp6BwAAQBAJ?q=fighting+was+fun&gbpv=1#f=false       

DESCRIPTIONS OF THE
BATTLE OF VERDUN

“Men were squashed. Cut in two or divided from top to bottom. Blown into showers; bellies turned inside out; skulls forced into the chest as if by a blow from a club.”

“You eat beside the dead; you drink beside the dead, you relieve yourself beside the dead and you sleep beside the dead.”

“I saw a man drinking avidly from a green scum-covered marsh, where lay, his black face downward in the water, a dead man lying on his stomach and swollen as if he had not stopped filling himself with water for days.”

“To die from a bullet seems to be nothing; parts of our being remain intact; but to be dismembered, torn to pieces, reduced to pulp, this is the fear that flesh cannot support."