The United States shot down four Iranian drones in the early hours of Thursday and bombed a ground facility in the south of that country, which led Tehran to attack a Washington military base in retaliation. These hostilities are the most serious since a ceasefire between the parties went into effect on April 8, after more than a month of attacks by the United States and Israel that unleashed a regional war that has caused thousands of deaths and disrupted the global economy. Find out: Iran accuses U.S.