The USA can barely handle one war at a time. Two wars against backward, third-world countries practically bankrupted them.
The US could probably beat any nation on Earth in one-on-one warfare, but all of them? No chance. For the past 70 years the US has gone up against 3rd world nations or backwards obsolete militaries and struggled against some of them (Vietnam and Iraq were nothing shy of messy). A battle against a large nation like China or India or a nation with a modern or sizeable military like Russia, Israel or a Western Europe state (like Germany, UK, France etc.) would leave them badly wounded.
Nah - Japan's military was crippled during the Potsdam Declaration and their navy was completely dissolved. They still have their Maritime Defense Force (think Coast Guard but with destroyers and submarines), but the treaty did not allow for an offensive fleet to be built; just a defensive fleet.
I still think there's a secret lab deep within Mt Fuji where a huge fleet of Gundam are being assembled. I don't think the treaty made mention of mecha being prohibited...