Without a means of enforcing bilateral trade agreements, Trump could simply change the terms of any deal down the road. The answer is to kick the U.S. out of the World Trade Organization, argues professor Kristen Hopewell.

When Trump threatened Brazil with punishing tariffs on Wednesday, there was no fancy mathematical equation trotted out this time as justification.

The 50% levy on goods was retribution for a perceived “witch hunt” against his political ally, Jair Bolsanaro, whose supporters attempted a coup two years ago.

This administration is bringing back the law of the jungle to international trade, according to trade expert Kristen Hopewell, and time is running out to salvage the remains of a post-war system of commerce that promoted stability and prosperity across the world.

“Trump is a totally unreliable negotiating partner,” she tells Fortune. “Any deal you strike with the administration is not worth the paper it’s written on.”