The First World War is an interesting case in point.
It was a war almost everyone wanted. They went off singing merry songs and 15 year olds lied about their age in hope of getting to share in the glory. "All Quiet on the Western Front" should be compulsory viewing for all (especially political leaders).
The US did stay out longer than most and actually only had troops there in numbers by May 1917 onwards and most of their fighting took place over the 1917/1918 winter. The US commanders were keen to demonstrate their ability and refused to share front lines with the Allies preferring to take on the Germans themselves - thus repeating the errors of 1915/1916. For the limited action the US forces saw they took ridiculously heavy casaulties - running full tilt into heavy machine gun fire till there was no one left to run; a party trick of the French and British for the previous three years.
Fortunately, by 1918, the Germans were on their knees and although the Front Line never really moved significantly one way or the other, by November 1918 they called and end to it. The Allies exacted heavy retribution on the Germans and gave rise to Hitler's accusation that these Germans were the November traitors; selling out their country. Paving the way for a re-match.
The moral - when people think war is a really good idea, duck.
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