1: The boots are so small because of Uncle Sam not having to wear them for so long. It represents the fact that America didn't have to go to war for so long and it allowed Uncle Sam to grow too big for his shoes.
2: It represents the fact that trouble was going on around the world and America seemed to be woken up by the fact that it was so abrupt.
3: America was not prepared and was "asleep" for a long time and had to attack fast, but wasn't prepared so it was "woken up".