Political map of North America & the Caribbean 04 April 1949 (North Atlantic Treaty): World War II left the United States and the Soviet Union as the dominant powers in Europe. Relations between the two began to deteriorate almost immediately, especially after US concerns over the Soviet installment of Communist regimes in Eastern Europe prompted President Truman to embark on a policy of "containment" (Truman Doctrine) by aiding the non-Communist regimes which surrounded the Soviet bloc. As this new "Cold War" heated up, the US and its European allies signed the North Atlantic Treaty, formally creating the NATO alliance to counter any potential Soviet attack.