1A great war or major conflict on a large scale. Rarely used alone in casual speech — mostly refers to World War I or World War II.
その村は大戦で焼け野原になったと祖父から聞いた。
I heard from my grandfather that the village was burned to the ground in the great war.
大戦を経験した世代は年々少なくなっている。
The generation that lived through the great war is dwindling year by year.
2An abbreviation of 世界大戦 ('world war'), used as in 第一次大戦 (WWI) or 第二次大戦 (WWII).
祖母は第二次大戦の直後に生まれた。
My grandmother was born right after the Second World War.