Germany’s unrestricted submarine warfare drew us in. Up until the ‘unrestricted’ policy was announced we were as mad at the British as the Germans because of their surface blockade of the North & Baltic seas. The only way Germany could retaliate was by a submarine blockade, and unfortunately, that inevitably meant sinking US ships.
One of the most interesting episodes of WW1 (or after) was US forces landing in Russia and kicking commie ass.
http://en.wikipedia.org/wiki/Polar_Bear_Expedition