Question: A Christmas tree was a type of alert area created during the tension that began in which year?
Sentence (from "Cold War"): Historians do not fully agree on the dates, but a common timeframe is the period between 1947, the year the Truman Doctrine (a U.S. foreign policy pledging to aid nations threatened by Soviet expansionism) was announced, and 1991, the year the Soviet Union collapsed.
Relevant evidence: Yes