There was a documentary or some kind of interview a while back on the History or Discover channel or somthing like that where they asked several Generals from around the world the same question. Almost all of them agreed that it would be almost impossible to invade the US successfully or destroy our nuclear arsenal It is virtually impossible to eliminate the US nuclear arsenal since it is based on a triad of land, air, and sea delivery systems designed to provide a counterstrike capability. The submarine-launched ballistic missiles in particular are widely accepted as the most survivable element of the US nuclear deterrent as a portion of it is always at sea. The land-based missiles too are difficult to eliminate, as they are in hardened silos in the middle of the country. Any adversary facing the United States would need to either be willing to absorb a nuclear attack or develop a ballistic-missile defense system currently beyond the scope of anything technologically feasible. 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 sealift 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 the amphibious-assault capability of the combined militaries of the world are simply too insignificant to get a beachhead on a coast. If they managed to go undetected, itself an impossible feat in light of modern surveillance capability, they still could not build up a force of any size before being pushed back into the sea. the amphibious-assault capability of the combined militaries of the world are simply too insignificant to get a beachhead on a coast. Thus, an invasion would have to come via a land border, with the terrain of the southern border (that with Mexico) being most conducive to military operations. However, the fact that the largest US Army armor base happens to be in Texas naturally would hinder such an attack. Going through the Canadian border—out West, to avoid the Great Lakes and St Lawrence Seaway—would be easier, although the invasion would then be limited to light infantry and would have trouble concentrating forces. In addition, it would fail to take over population centers or other important strategic points, since it is mostly national parks out there. 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. And if for some odd reason they do land and manage to get a foothold they still have to worry about all the armed citizens we have.