The US' greatest defense is the Atlantic and Pacific Ocean. It would be impossible to launch any sort of conventional invasion by sea in force. Even if the navies of the world combined beat your navy, you have a couple thousand nukes to spare for tactical strikes. I assume those can be used on naval targets as well as ground targets, after all. So, no one would be able to invade you.
But you'd lose in the end, of course. Can't make war without money, and without trade, your coffers run dry. Your populations are also unwilling to fight wars. The US wouldn't be able or willing to lose the number of soldiers Russian or Chinese governments would be able to sacrifice, so your own invasion attempts would be thwarted, too.
And unlike Europe, or Asia, or Africa, you'd not be able to recover socially or economically from such a war. Especially with American GenX'ers and Millenials being indoctrinated into believing that the collapse of government means the US turns into the
Fallout universe. Americans just aren't built for cooperation anymore and don't posses enough of a cohesive
society to rebuild. The rest of the world will easily influence you into a civil war state, leading to seccesionism, wide-scale civil unrest and radicalism. And every time an American tries to tell you that you're being manipulated by foreign propaganda from...say, China, everyone else will simply dismiss it as "
China! China! China!" alarmist nonsense.
And then Canada annexes you without a single shot fired.
Fame is not flattery. Respect is not agreement.