Jonathan Caulkins, Angela Hawken, Beau Kilmer, and Mark Kleiman have a very helpful primer on marijuana legalization. One important point:

In theory, the Federal government could counter state legalization by hiring or reallocating enough DEA agents, judges and prison cells to preserve the status quo ante, leading to no change in production costs, prices, use or any other outcome, except that the costs of enforcement would then be borne by Federal rather than state taxpayers.

In practice, that is extremely unlikely. State and local police forces employ about 750,000 sworn officers; the DEA has only 5,000 special agents. In 2010, state and local law enforcement made 97 percent of the more than 800,000 marijuana-related arrests nationwide. Most were possession arrests, which Federal agents very rarely make, but even among sales and distribution arrests, state and local agencies accounted for roughly 90 percent of the total outside border regions. So as a practical matter, state-level legalization would dramatically reduce enforcement risk, at least for those handling quantities too small to attract Federal attention. (Many U.S. Attorneys’ offices don’t bother to prosecute cases involving less than several hundred pounds of marijuana.)

Another bit worth highlighting:

[T]he Federal government would face a difficult choice. It could effectively thwart a thoughtfully designed repeal-and-regulate scheme like Washington’s, and it could prevent Colorado’s Department of Revenue (the agency CO-LA empowers to regulate the new marijuana industry) from producing regulations that take Colorado in the direction envisioned by I-502. It would have a much harder time nullifying a loose regulatory scheme and, short of massively expanding the DEA payroll, could do essentially nothing to stop a legalize-only action such as the Michigan amendment.

In short, the Federal government may be incapable of stopping legalization, and the primary effect of an aggressive response might be to push legalization toward a relatively unregulated open market.

The article goes on to explain why "a strict non-interference policy on the Federal level might also be untenable."

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