Invade the U.S. ? Yeah it can be done, and if every nation joined in, just the sheer amount would prove a difficult task to defeat, and invasion success is most likely, but it would become a battle of attrition by then, with the invaders paying a very high price. Let's look at some of the issues with an invasion.
To propose a straight out attack via land and sea, you would have to either position troops beforehand(sneaking in through freighters via Canada/Mexico), or land them alongside your ships. This itself is near impossible, as a large enough land force would be difficult to sneak over and keep hidden, much less fully supplied and equipped. An immediate attack by sea is highly impractical, as the U.S. has many long range strike options(not to mention their navy) as well as good intelligence agencies and satellite systems for detections of such a fleet. A sea invasion would only be practical if some of these options were eliminated. Also, any initial attacks would be wise to also have forces stationed to take out the U.S. bases across the world, disbarring a 'full' scale invasion, as well as increasing likelihood of discovery, for having such forces near every U.S. base on the globe would be a bit suspicious. If that is not done, then it will become a global war with near countless fronts, and nations pulling back their armies to defend their homelands instead of invading the states.
As for points of attack, the west coast is impractical if you aim to expand to the east coast as well, for the cascades provide a strong barrier, and all major roadways through them will be deathtraps(guerrilla tactics, artillery, or just destroyed roads), limiting any reliable/timely transportation across to air travel, which will hinge on air superiority. To hit the east, as mentioned, the number of towns and roads make it a tactical nightmare for the military to control and expand. By land from north or south you will hit the same issues if you aim along the western or eastern edge. Cutting through the middle can divide the nation and cause confusion, but will also leave the invasion force highly exposed, as well as their logistics line behind them. The south provides the largest insurgent threat as more rednecks and gun touting rights activists will be there, making the north a bit more viable. But, the most important factor to remember is that such an invasion will unite the populace against you, for announcing an invasion(in hopes of scaring the populace to submission, or whatever other reasons) would eliminate a sneak attack option, whereas a sneak attack will cause all the citizens to distrust and hate the invading nations.
In contrast to an all-out invasion, taking over or blockading key economic points have been brought up. These would indeed eventually cripple the nation(if the economic system remained in effect at all during such an incident), but also cripple the economy of the world once more. Remember that a central cause for the current economic issues did indeed originate from the states, but even then, a majority of the businesses in the world still manage their money and trade through wallstreet/dow jones. Disrupting a central hub of trade and business in order to cripple the states would cripple the world just as badly. Also, although a majority of production is done outside the US, most of those places are run and owned by companies based in the states, and with them cut off from their headquarters - and subsequently their funding - it is doubtful they will continue to operate, now just imagine the fallout from factories around the world shutting down. Hell, even if it was a full invasion instead of a targeted economic attack, these same economic effects would be felt around the world.
In further points, although the US military is somewhat small in numbers, their technology and resources are second to none. It is also good to note that there are many ex-military citizens out there, whether retired or those that served for a short time instead of career, most of whom likely have kept weapons and trained up. The police forces and militias are also not to be underestimated, for although they might seem weak, they are trained, organized, and experienced. There is also the fact that, as the ones being invaded, they have the homefield advantage, whether its in knowing the terrain, or fighting to defend their homes, which is a zeal no invader can match.
For now, this is good, feel free to pick, I'll respond when able. For those who may be curious, I am an American.
