Canada's Justin Trudeau may legalize marijuana. That could impact US drug policy.

As Canadian Prime Minister Justin Trudeau and the country’s Liberal Party move forward with their big agenda following last year’s election, the US’s northern neighbor could soon undertake an enormous change in drug policy: marijuana legalization.

The policy was a big part of the Trudeau and the Liberals’ campaign: “We will legalize, regulate, and restrict access to marijuana. Canada’s current system of marijuana prohibition does not work. It does not prevent young people from using marijuana and too many Canadians end up with criminal records for possessing small amounts of the drug.”

If marijuana were legalized in Canada, it would be a first among wealthy nations. In the US, four states and Washington, DC, have legalized pot, but it’s still illegal at the federal level. The only other country to fully legalize marijuana is the tiny developing country of Uruguay. And although some countries — the Netherlands and Spain, in particular — have relaxed enforcement of their marijuana laws, none in the developed world have outright legalized it.

But this wouldn’t just be an important milestone for Canada and the world; it could also send ripples across the international system of drug policy. That’s because drug policy is tied not just to each country’s individual laws, but to a network of treaties that effectively make the war on drugs a global effort. Marijuana legalization in Canada would act as the most high-profile rejection of these treaties, sending an important signal of the changing times as the international agreements come under a critical review.

From the 1960s through the 1980s, much of the world, including the US, signed on to three major international drug policy treaties: the Single Convention on Narcotic Drugs of 1961, the Convention on Psychotropic Drugs of 1971, and the UN Convention against Illicit Traffic in Narcotic Drugs and Psychotropic Substances of 1988. Combined, the treaties require participants to limit and even prohibit the possession, use, trade, and distribution of drugs outside of medical and scientific purposes, and work together to stop international drug trafficking.

There is some debate about whether these treaties stop countries from decriminalizing marijuana — when criminal penalties are repealed but civil ones remain in place — and legalizing medical marijuana. But one thing the treaties are absolutely clear on is that illicit drugs aren’t to be allowed for recreational use, and certainly not for recreational sales. Yet that’s exactly what the Liberal Party has promised to allow.

(For those curious, the US has remained in accordance of these treaties despite four states’ move to legalize with a clever argument: It’s true four states have legalized pot, but the federal government still considers marijuana illegal, so the nation is still technically in obedience even if a few states are not.)

So Canada’s decision to legalize pot — if it comes, and that’s still unsure — would be the most high-profile rebuke of the international treaties since they were signed. Not only is Canada an internationally active, developed nation, but it’s a relatively large country — larger than all the states to have legalized so far and Uruguay combined.

In theory, Canada could face diplomatic backlash if it legalizes pot. But who would lead that effort? The US has been the de facto enforcer of these treaties over the years. But it likely wouldn’t tempt an important ally, and trying to criticize Canada for legalization would only expose America’s hypocrisy for allowing four states and DC to legalize.

Chances are, then, that Canada will be able to legalize marijuana, and potentially do so without any global repercussion. That will send a big signal to other countries — that, at least when it comes to marijuana, these treaties no longer hold the weight they once held. It could, then, expose a huge hole in the treaties, making more nations comfortable with the idea of legalization.

Such a move would come at a very crucial time in international drug policy: After the UN’s special session on drugs in 2016, drug policy reformers are putting more and more pressure to reform the global drug control regime. Canadian legalization would give these reformers an opening by showing that if the treaties aren’t changed, they will soon be effectively meaningless as countries move ahead with their own reforms, treaties be damned. And such treaty changes could open other countries, including the US, to their own reforms.

Now, it’s possible Canada will ultimately decide not to legalize — the treaties, for instance, could cause the new Liberal government to fear an international backlash and back off. But if Canada does move forward, it will not just change Canadian drug policy, but potentially force a big shift in the international stage.