I don't think anybody believes that he will "invade" the united states. I don't think that anybody would be stupid enough to attempt that.

The USA is demoralized currently, nothing would rally us faster than an enemy on our home soil. The citizenry of the United States is currently a sleeping giant, do they really want to wake us up? We can be brainwashed into any common goal (think about 9/11) if there is a perceived threat.

Besides that, he wouldn't be able to break through with his pathetic navy with our submarines hunting the pacific like sharks.

What may happen, though, is that they will invade and take South Korea. That would lead to lots of dead civilians and American soldiers.