The U.S. is technically in violation of a couple United Nations (U.N.) treaties regarding marijuana. The first treaty, the 1961 Single Convention on Narcotic Drugs, was designed to get the whole world on board with the United States' drug war. It's main purpose was to force other countries to criminalize drugs and actively pursue offenders or be threatened with sanctions. It also established the International Narcotics Control Board, which polices foreign governments to monitor compliance.
While marijuana is prohibited under the treaty as a member of the worst classification, Schedule I, it does leave room for medical uses.
CONTROL OF CANNABIS
1. If a Party permits the cultivation of the cannabis plant for the production of cannabis or cannabis resin, it shall apply thereto the system of controls as provided in article 23 respecting the control of the opium poppy.
Though it seems the U.S. may still be in violation.
NATIONAL OPIUM AGENCIES
1. A Party that permits the cultivation of the opium poppy for the production of opium shall establish, if it has not already done so, and maintain, one or more government agencies (hereafter in this article referred to as the Agency) to carry out the functions required under this article.
We can probably assume that whatever state agencies control the licensing in medical marijuana states will qualify as the Agency spoken of in the treaty. Even Colorado and Washington didn't disqualify us from this treaty as it fails to specify what uses a country that allows cannabis production is restricted to or from.
So it seems we're not in violation of the 1961 Convention, but what about the 1988 United Nations Convention against Illicit Traffic in Narcotic Drugs and Psychotropic Substances? That's the treaty that declared open war on narcotics and encouraged the genocide of psychotropic plants.
MEASURES TO ERADICATE ILLICIT CULTIVATION OF NARCOTIC PLANTS AND TO ELIMINATE ILLICIT DEMAND FOR NARCOTIC DRUGS AND PSYCHOTROPIC SUBSTANCES
Very stern title, all caps even. This is the treaty that set the stage for all of the video you see of government agents setting fields of cannabis or poppy ablaze with flame throwers. It was an evil that had to be eradicated.
1. Any measures taken pursuant to this Convention by Parties shall not be less stringent than the provisions applicable to the eradication of illicit cultivation of plants containing narcotic and psychotropic substances under the provisions of the 1961 Convention, the 1961 Convention as amended and the 1971 Convention.
There appears to be a loophole left open here. If you notice, there is very stern warning against the "illicit" cultivation of plants. A state licensed business would be involved in "licit" cultivation.
2. Each Party shall take appropriate measures to prevent illicit cultivation of and to eradicate plants containing narcotic or psychotropic substances, such as opium poppy, coca bush and cannabis plants, cultivated illicitly in its territory. The measures adopted shall respect fundamental human rights and shall take due account of traditional licit uses, where there is historic evidence of such use, as well as the protection of the environment.
Perhaps that's not a loophole after all. The treaty seems to have purposefully left the door open for legal cultivation of cannabis, poppy, and coca. Throwing protection of the environment in at the end almost seems as if we should be compelled to grow hemp, at least for industrial purposes. It is the most "green" building material, clothing material, bio-fuel, etc.
So it would seem we aren't in violation of any treaties, and neither is Uruguay. Further legalization will also be well within the bounds of those treaties, as long as we don't import or export. Since we've been importing marijuana through the black market before legalization efforts started making headway, the birth of legal markets within the U.S. only makes us more compliant to the treaties.
In the words of Uruguayan President Jose Mujica, "We are trying to invent a path, picking up experiences as we go. There are people who say that you can't experiment. ... That condemns you to failure."