Remember in this argument, I am in no way trying to prove that the USA is better that the rest of the world. I am simply saying that the USA could defend itself from the entire world. First of all the naval defence of the USA could defend from multiple countries attacking at once. Second of all, if the defences were to be breached, and another country managed to get through to Mexico or Canada, the USA wouldn't sit and wait for the enemy to get to the borders. Even so Tlthe USA could find off a huge military attack force, hence planes, ships, soldiers. The outcome of it all is unclear but I think the USA could pull it off.
The United States of America would never win against whole entire world. We are clearly out numbered with are army of over 100 million people. Even with are technology, we wouldn't last long due to us fighting with every countries army. We would be pinched in by every country. Also, if we were to go to war we depend on a lot of imports from other countries. They would cut all those imports from us and take all of our resources leading to very bad situations.