Tariffs are Trump's weapon of choice in on-again, off-again trade wars with Colombia, Canada, Mexico, China, importers of cars, importers of steel and aluminum

US businesses bringing steel and aluminium into the country now have to pay a 25% tax on them, as the EU says it will retaliate.

Trump slapped similar tariffs on EU steel and aluminum during his first term in office, which enraged European and other allies. The EU also imposed countermeasures in retaliation at the time, raising tariffs on U.S.-made motorcycles, bourbon, peanut butter and jeans, among other items.