Well, lets just put it this way, if you slashed the current defense budget by a quarter, it would still be the highest one in the world (China's would come a close second).
Now North Korea still has the largest army per capita in the world, but that really doesn't stand for much. Most analysts believe is just a smoke screen for a state that acts similarly to the mafia.
Also, because of the big dog status of China in the region, you would first have to cut ties with China first, or Korea would have to reach hegemony in the region, any of these seem likely within the time frame of Homefront.
A fourth problem follows:
North Korea, population: 24,051,21
South Korea, Population: 48,875,00
China, population: 1,339,724,852
USA, population: 313,370,000
Even both Koreas united... might not be able to yield out the logistics or the number of troops for a full blown invasion over a population over three times it's size (specially if spread thin, wouldn't China just invade them then?).