The U.S. military vowed to blockade all Iranian ports starting Monday, part of efforts to force Tehran into agreeing to open the crucial Strait of Hormuz and accepting a peace deal. Iran responded with threats on all ports in the Persian Gulf and the Gulf of Oman, taking aim at U.S.-allied countries.

That set the stage for an extraordinary showdown that contains serious risks for the global economy and raises the specter that a ceasefire that is currently holding could collapse and the war could resume. Talks aimed at permanently ending the conflict — which began Feb. 28 with U.S. and Israeli strikes on Iran — finished without an agreement this weekend, and there has been no word on whether negotiations will resume.