re: How successful would a country be invading the U.S.?

Here's how it would go:

1) Foreign enemy decides to invade America.

2) Vastly superior US military obliterates any forces that get within 100 miles (if that close) to the coast.

The end.

Considering your edit - if they DID make it to the land, then our national guard would be deployed. Our police have military grade weapons. And the populace is heavily armed, though I think outside of the Midwest, South, and Texas, this would be overrated.

Los Angeles, San Francisco, and Seattle are too big to occupy. A good laugh thinking they'd take San Diego. Maybe they get through Oregon, but the terrain is difficult.

Then what? Traverse the fricking desert or Rocky Mountains?

East coast is heavily populated and too big to occupy any major cities. Lots of military presence there as well. The South will shoot them on sight. Florida has weather and terrain issues.

Probably best case is to somehow get through Canada into Wisconsin or Michigan. Secure the lakes. Launch an offensive to cut off Chicago, Detroit, and Buffalo.