Unlike some posts I saw as I scanned this thread, I mean no disrespect to anyone here when I say that a successful invasion of the United States by anyone, especially North Korea, is damn near impossible. Now, I'm not just an over-patriotic fool. Being into military science and military history, though still not (yet) a professional, I have been able to look around and understand how such an invasion would work. I can honestly say that I don't see a successful invasion of the US being possible by any nation for the next 50 years, and at the least, if we are generous and give the enemies of the US the benefit of the doubt to say they get very powerful very quickly, 20 years.
Take China for example, being currently the nation most likely to get in a fight with the US. China's total military size, including reserves, active personnel, and paramilitaries, is about 3.45 million personnel. The US has a total of about 2.45 million. So, yes, they outnumber us by damn near 1 million exactly. That's about 1.4 Chinese soldiers per American. Then, take into account the total populations of each country. The US has near 310 million people to China's 1.3 billion. So, China certainly has the numerical advantage, since they could quickly build their numbers and industry with untrained manpower (training takes time, of course, so if they need to really sap their numbers to use human-wave, that is not ideal for time's sake).
The United States would, initially, have a vastly superior conventional force against the Chinese with better trained soldiers and higher quality equipment. In the ideal, rapidly moving modern war that is very likely in this day and age, the real thing to consider is the order of battle of a nation to start. Only in a total war scenario in which populations and industries are mobilized does one really consider the rate at which material and personnel strength is replenished. In addition, China's troops are not fully mechanized or equipped for proper transportation. US troops are, of course, very well equipped and mechanized.
The real issue is the fact that the US and Chinese are separated by 5,000mi of Pacific Ocean. Currently, the United States Navy currently has a total of 11 aircraft carriers, ten Nimitz Class and one Enterprise Class. Now, not all carriers would be in the Pacific Theater to fight at first, but the Navy has fleets in the Pacific Ocean, South China Sea, and Indian Ocean/Persian Gulf, or rather those regions in general. All such fleets would be able to challenge Chinese naval power. Additionally, American air forces would likely have access to bases, as it already does, in Japan, which would also be able to contribute the JASDF (Japanese Air Self-Defense Force) to the fight. The US also has footholds in South Korea, which is also a US ally and would be able to put its forces (however inferior to China's) into action.
North Korea would likely support China, but its forces would be negligible compared to either superpower. Now, given the US presence nearby, the Chinese would surely have something to offer. Unfortunately, the Chinese Navy is as yet insufficient to fight ours (I have heard that they are just now building their first carrier. I'm not sure how true this is and frankly it is surprising, so I don't yet believe it), though I don't have the specifics on the Chinese Navy quite yet. In any case, the addition to American carriers would come in the form of the second naval vessel which still serves a large purpose in modern strategy: submarines. US nuclear submarines have only gotten stealthier, and as a result, deadlier since the Cold War. Aircraft carriers are used to project firepower over long distances, and submarines are used to deny an enemy access to a region. Submarines are to naval warfare what parrying is to fencing. They prevent the enemy from using his projection of power.
This isn't even taking into account NATO. China, for purposes of remaining unrestrained by permanent alliances, is in none that I am aware of. However, America has NATO, and Britain, the nation ranked second in the world for power projection, would likely come to the aide of its ally.
So, without control of the Pacific, China would be unable to transport troops overseas, thereby stopping the invasion before it even begins.
But, Homefront has North Korea, not China. I think you see my point, though. North Korea is much, much weaker than China, and the if the US is not likely to be beaten by China, then North Korea will need to do a lot of improving, and the US a lot of falling into the crapper, before something like this is even remotely plausible.