At the very least, Washington and Colorado have laid a Gordian knot on the President's desk.
How, exactly, does the US respond, given that a recent Gallup poll finds that 63 percent of Americans want the federal government to leave the two states alone? Moreover, legal experts say, the laws are not at their core contradictory to federal policy.
Both state schemes will continue to regulate marijuana in ways designed to curtail, not promote, its use. In Colorado's case, tax revenues will go to local school districts. In Washington, police will be able to pull over stoners and prosecute them for intoxicated driving if they've had too much to smoke.
The legal issues are complex, and any response by the Obama administration could have broad policy repercussions on everything from enforcement priorities to how it would affect international antidrug treaties.
"Should the Justice Department prevail, it would raise the possibility of striking down the entire initiatives on the theory that voters would not have approved legalizing the drug without tight regulations and licensing similar to controls on hard alcohol," writes the New York Times' Charlie Savage.
So far, the only action the federal government has taken is US Attorney Jenny Durkan's stern warning to Washington residents that "growing, selling, or possessing any amount of marijuana remains illegal under federal law."