The United States announced Monday it would halt sanctions waivers for Iran’s Fordow plant, ending a key part of a landmark nuclear deal after Tehran said it had resumed enrichment activities. The move is intended to end Russian and European cooperation with Iran on the once-secret site, which was supposed to become a civilian research center under the 2015 agreement. Secretary of State Mike Pompeo pointed to Iranian President Hassan Rouhani’s announcement of renewed activity at Fordow — a series of steps taken by Tehran as it presses European allies to make good on sanctions relief promised for compliance.