The states that legalized marijuana can simply stop cracking down on the illegal sale of marijuana, making it de facto legal, even though it's de jure illegal under federal law. If the feds don't like it, they'll have to devote more resources from the DEA or US Marshals Service or the ATF to enforce the laws themselves. Let's see if the GOP-controlled House is willing to increase government spending in order to enforce the anti-drug pet initiative of their evangelical base.