President Trump has said he views tariffs as an important tool in his efforts to reshape the landscape of global trade. And has used them repeatedly in America’s escalating trade war with China.

This week, however, the president threatened to use tariffs in a way that caught most people off guard — as a bargaining chip to pressure the Mexican government to crack down on migrants heading to the U.S.

Whether or not the 5 percent tariff actually goes into effect on Monday, Trump’s threats have spooked markets, rattled the business community and sparked a backlash from Republican lawmakers.