Sentence:  A visibly frail Roosevelt played his usual role of mediator between the pugnacious Churchill and the scowling tyrant Stalin. All three leaders left Yalta believing they had obtained the assurances they needed for the postwar world. Churchill rebutted any and all suggestions that the British <WSD>ring down the curtain</WSD> on their empire. 
Meaning of the word marked with <WSD> tags:  To end something.