Final answer:
Winston Churchill's line about the "iron curtain" descending across Europe exemplifies his purposeful announcement of the start of the Cold War in his speech.
Step-by-step explanation:
In his speech, Winston Churchill declared that an "iron curtain" had descended across the Continent, dividing Europe into two halves: one under Soviet domination and the other free. He emphasized that the Communist parties in Eastern Europe had gained power beyond their numbers and were seeking totalitarian control. This line exemplifies Churchill purposefully announcing the start of the Cold War by highlighting the divide between democratic and Communist nations and the threat of Soviet domination.