The 1948 Berlin Blockade did not escalate to war, largely because Truman and Stalin did not want another world war so soon. Atomic weapons played no obvious role. The fear of atomic weapons did not deter Stalin – who lacked such weapons – from initiating the crisis. They also played no role in ending the crisis, since Truman made no explicit threat to use them. Instead, the airlift defeated the blockade, and Stalin ended the crisis. In contrast, nuclear weapons played a clearer role in the 1958–1962 Berlin Crisis. Both sides now had a second-strike capability, but that deterred neither Khrushchev from initiating the crisis nor Kennedy from considering nuclear war. Nevertheless, the possibility of a nuclear war made each leader more prudent. As the crisis evolved, large numbers of East Germans started crossing into West Berlin. Ulbricht, the leader of East Germany, pleaded with Khrushchev to do something. Khrushchev permitted the construction of a wall (although Kennedy may have signaled his non-opposition). This ended the refugee flow and, therefore, the immediate crisis. The Berlin Crises intensified the Cold War. They led to the formation of NATO and the Warsaw Pact, as well as the division of Germany into two states.