Canada’s Prime Minister Justin Trudeau has built a huge part of his super progressive image around his focus on legalizing marijuana.

He talked about his plans to legalize throughout his campaign, and toward the end of 2015 he issued an official mandate calling on lawmakers throughout the country to begin the process of legalizing pot in earnest.

But now, it seems his THC train may be running into some serious roadblocks.

The biggest challenge facing the young Prime Minister’s program is international treaties.

In particular, Canada is a part of three international treaties, all of which stipulate the criminalization of cannabis.

The three treaties are:

The Single Convention on Narcotic Drugs of 1961, which was amended by the 1972 Protocol

The Convention on Psychotropic Substances of 1971

The United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances of 1988

A briefing not prepare earlier this week for Trudeau and picked up by the Canadian press explained that “all three require the criminalization of possession and production of cannabis.”

“As part of examining legalization of cannabis possession and production, Canada will need to explore how to inform the international community and will have to take the steps needed to adjust its obligations under these conventions,” the briefing said.

Some are speculating that Canada—along with several other countries taking rapid steps toward legalization, most notably Mexico—may use the upcoming United Nations General Assembly to make the case for legalization.

The next scheduled Assembly is in April, and it will feature a special session focused entirely on international drug-related concerns that could provide an ideal venue for such a discussion.

“There’s no international treaty where you cannot make reservations,” said international law expert Errol Mendes.

He said the key to Canada being able to move forward with its pro-pot plans is to convince the international community that there are good reasons for abandoning global anti-cannabis rules and treaties.

“The problem is the government having to explain why it’s doing it, why it feels it has to do it, given the conviction Prime Minister Trudeau has said it’s a failure in terms of enforcement in almost every respect and is driving up the crime rate in some parts of Canada.”