President Trump is reportedly mulling an executive order to withdraw the United States from the North American Free Trade Agreement, a major trade deal with Canada and Mexico that reshaped broad sections of the U.S. economy after going into effect in 1994. But it might not be as easy to get out of NAFTA as Trump may think.

The president’s aversion toward multilateral trade agreements placed him in a similar ideological camp as pro-Brexit voters in Britain, who narrowly won a referendum last year to withdraw their country from the European Union. Trump celebrated the result at the time and claimed he successfully predicted it, even referring to himself as “Mr. Brexit.” For him and his supporters, the surprise result across the Atlantic showed their upset victory could also be possible. It additionally demonstrated a broader populist backlash against establishment institutions.

Britain’s experience also illustrates how Trump might not be able to carry out his own Amerexit from the free-trade agreement he’s so frequently criticized. To understand the difference, look no further than Article 50 of the Treaty on European Union, which establishes the legal mechanism by which a country can withdraw from the European bloc.

1. Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements. 2. A Member State which decides to withdraw shall notify the European Council of its intention. In the light of the guidelines provided by the European Council, the Union shall negotiate and conclude an agreement with that State, setting out the arrangements for its withdrawal, taking account of the framework for its future relationship with the Union. That agreement shall be negotiated in accordance with Article 218(3) of the Treaty on the Functioning of the European Union. It shall be concluded on behalf of the Union by the Council, acting by a qualified majority, after obtaining the consent of the European Parliament. 3. The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period.

Setting aside the legalese, Article 50 is essentially a ticking time bomb with a two-year fuse. First, a country formally notifies the European Council of its intent to withdraw. This may seem largely procedural since no country would decide to leave without a referendum or parliamentary vote of some kind. But the intent to withdraw isn’t really about informing European leaders; it’s about starting the two-year countdown. Once time is up, the country is out.