The only way for a foreign power to defeat the USA

The US - at least for now - will dominate militarily against any adversary.

They can't take us out through any kind of conventional war.

Biowarfare could do the trick, but it's too extreme a bet. The US might be able to defeat the illness before it does enough damage, or the illness could spread outside the US and hit them too.

The way I see it, the only way to defeat the US is an internal blitzkrieg/flash mob of existing sleeper cells, loyalist immigrants and politically like-minded US citizens. They would cause chaos at large gatherings of people, while other teams would attack infrastructure and communications hubs - all over the nation.

Armed US citizens think that we would be well-equipped to fight back, what with our own weapons, number of ex-military citizens and our current natl guard and home-based military. However, history shows that when chaos is in the air and people are in fear, their plan A is to defend their own castle. They'll stay home, waiting for a bad guy to stroll by for them to shoot. This creates a window of time in which nothing is guarded very well, where a foreign army could just walk right in.

If, say, China, Mexico and Iran decide to ally together to take us out, they could activate the small percentage of their millions of immigrants currently living here to carry it out. Might only take 10k people or less to do the trick. It wouldn't be too hard for them to find useful idiot murderers within our native political left either.

This open borders bullshit has to be stopped and reversed asap. Any adversarial nation would have to be mega-retarded to not have already exploited our idiotic system to smuggle in their own agents.

Anyway...I fully expect that if a major player nation on the world stage really wants to take us out, that's how they would do it.