Why did the U. S. have the least amount of casualties?

The US Army didn't participate in the Western Front until October 1917 when they fired their first shell at the German lines. The 4 American divisions involved did very little in those battles. It wouldn't be until May 1918 that the US would take on its actual first offensive. During the next 5 and a half months the US would sustain 254,000 casualties (dead and wounded). If we extrapolate that out to the entire war thats 2.2 million, about the same as the UK.

That's despite being able to learn from their Allies; the UK and France.

But that's unfair to the US as well. Starting in May of 1918 it played a key role; perpetual offense. American troops were well trained and poorly led. They made the same disastrous mistakes that the other forces did, but their learning curve was steeper.

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explanation:

the answer is a glaciers carved a large valley that later flooded and became puget sound