Trump uses tariffs as a key tool to force changes to U.S. trade relationships, but his on-again-off-again approach has roiled markets and stoked uncertainty.

The U.S. started imposing tariffs by letter to a variety of countries Monday, pairing new rates with an explicit warning that even those could change at any time. Why it matters: After months of threats,

Trump on April 2 proposed a 20% tariff for EU goods and then threatened to raise that to 50% after negotiations did not move as fast as he would have liked, only to return to the 10% baseline. The EU has 27 member states, including France, Germany, Italy and Spain.