The short answer is no, and the chances are not even likely…remote is even too brave as description.
First of all you have to assume that the uS has lost its nuclear capability…let’s assume that.
So, once the nuclear capabilities are down, what could an invasion of the US look like?
The US is the sole country in the world that has the capability to project force across the globe on a large scale. The combined military air- and sea-lift capability of the rest of the world would be insufficient to even get a foothold on the continental United States. The amphibious assault capability of the world’s militaries, excluding the United States, is simply too small.
That means the adversary would have to seize and use civilian aircraft and ships not designed for nonpermissive environments. These ships would require secure bases in Canada and Mexico, since they lack the capability to deliver forces onto unimproved shores. Thus, any attempted invasion of the US would first look like a rather motley caravan of vulnerable civilian ships and aircraft.
If these forces managed to avoid US attacks and build up, they could then launch an attack over land.
In other words, it is bordering on impossible.
The primary problem here is geography. Just as the vast Russian steppe swallows armies, so would the oceans that surround the US. No matter the manpower or armament, it must be delivered across the Pacific and Atlantic in order to be brought to bear. This is where US naval and air power would destroy any adversary, far before they sullied the US shore.
And this is where you meet the second primary problem, which is technology. There are not enough aircraft carriers and amphibious warfare ships in the combined navies of the world to force an entry past the US Navy. There are not enough attack fighters to gain air superiority against the US Air Force. This is how amazingly out of balance the military might of the world is today.
Go ahead…make their day…to paraphrase Clint Eastwood.