Psychological warfare would be the way to go, the power of the US could be its downfall if someone were to identify and undermine the foundations of national unity, e.g. cause fear (a dozen trained "serial killers" travelling around the country) fuel paranoia (come up with conspiracy theories and propagate them) spread drugs/weapons/crime, this will all contribute to destabilization of social order, then you take out the president, the vice president and whoever comes next all at once (a challenge I know) to undermine the legitimacy of government.
Then in theory the socio-political environment should be right for factions to form, after which you continue causing trouble here and there to further escalate the situation until eventually the United States are no longer united.
Finally you get the fuck out of there before the other big fish smell blood in the water.
Edit: This process could obviously be refined, it's just an example.