What if the USA entered WW2 from the beginning?

Let's presume that the Americans declare war on Germany together with the UK and France. The answers to the following questions interest me:

1. Would USA as France think that it will be trench warfare again and bury its forces along the Maginot's line, or would it pursue a more active form of fighting and try to invade Germany's and Italy's western borders (together with BEF and French forces, of course)? If the answer is the latter, would the forces be sufficient to defeat Germany by 1940?
2. Would the the presence of a large military force in the West make Hitler delay Operation Barbarossa or give up on it completely?
3. How would this affect things in the Pacific? With the USA now on military alert, would Japan still attack Pearl Harbor?