Conquering the U.S. today would require an unconventional & asymmetric approach. One possibility could be intentionally dragging the U.S. into multiple seeming

Conquering the U.S. today would require an unconventional & asymmetric approach. One possibility could be intentionally dragging the U.S. into multiple seemingly unrelated wars along several fronts (Iran moves to blockade the Straight of Hormuz, Saudi Arabia bans export of oil to the U.S. and 'dares' them to come take it back, and China shuts off all exports of goods to the U.S., while on their campaign to 'liberate' South Korea). Each of these actions are designed to seriously wound the U.S. economically thus inviting a military response. While the U.S takes the bait to engage in these 3 conflicts and overextends herself (the U.S. military is built to fight just 1.5-2 wars), a group of sleeper agents who have risen to positions of considerable power in the House and the Senate begin the next phase by obstructing the war effort (denying funding to the troops), and generating significant domestic dissent (raising taxes considerably, while simultaneously cutting spending to subsistence levels (no more food stamps, delay social security checks to make grandma go hungry). By this point, the country has no access to oil beyond our strategic reserves, the economy is in shambles, rich people are pissed by the 80% war tax, and poor people are pissed because they're not getting anything from it. Now we enact phase 3 and send in the Canadian Mounties. They'll be greeted with open arms by all Americans.