[PAR] [TLE] Christmas tree (aviation) [SEP] A "Christmas tree" was a type of alert area constructed by the United States Air Force for the Strategic Air Command (SAC) during the Cold War.  Oftentimes, bombers or tanker aircraft were stationed next to a readiness crew building (RCB), also known as "mole hole" facilities.  The alert apron, also known as an alert ramp, received the name "Christmas tree", because in planform it resembled a tree of the same name. [PAR] [TLE] Cold War [SEP] The Cold War was a state of geopolitical tension after World War II between powers in the Eastern Bloc (the Soviet Union and its satellite states) and powers in the Western Bloc (the United States, its NATO allies and others).  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.
[PAR] [TLE] Christmas tree ( aviation ) [ SEP ] A " Christmas tree " was a type of alert area constructed by the United States Air Force for the Strategic Air Command ( SAC ) during the Cold War . Oftentimes , bombers or tanker aircraft were stationed next to a readiness crew building ( RCB ) , also known as " mole hole " facilities . The alert apron , also known as an alert ramp , received the name " Christmas tree " , because in planform it resembled a tree of the same name . [PAR] [TLE] Cold War [ SEP ] The Cold War was a state of geopolitical tension after World War II between powers in the Eastern Bloc ( the Soviet Union and its satellite states ) and powers in the Western Bloc ( the United States , its NATO allies and others ) . 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 .
A Christmas tree was a type of alert area created during the tension that began in which year?
A Christmas tree was a type of alert area created during the tension that began in which year ?
1947
1947