4,355,000 US Soldiers fought in WWI
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Like me, I think you are guilty of shooting from the hip.
2,000,000 US soldiers fought in FRANCE. There were 4,000,000+ US troops in uniform but not all of them "fought".
I think to make a fair comparison you must compare like figures. I am choosing to compare soldiers in combat zones. 150,000 US troops in a COMBAT ZONE (Iraq) vs 2,000,000 US soldiers in a COMBAT ZONE (France, the US did send a small force to Russia as well). As for the Iraq War, if you include support personel the figure will be higher than 150,000.
126,000 US soldiers died in WWI
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This site says 113,000 American dead.
http://www.nps.gov/elro/glossary/world-war-1.htm
My encyclopedia Britannica says 126,000. The 126,000 includes deaths caused by wounds so if a soldier died of an infection 4 months after being wounded then his death was included as a combat death. I suspect the differences in the figures are wounded who later died because of complications due to their wounds.