This history of NATO concentrates on the differences within the alliance, particularly between the US and its European partners. NATO's war against terrorism began on September 11, 2001. Invoking Article 5 was a fitting response to the assault on the United States, but the spirit did not last long. Within a few weeks, old fissures within the alliance re-emerged, threatening once again to dissolve an entity that had survived over half a century. In the first two generations of NATO's existence, the Cold War with the Soviet Union had been the major purpose of its existence. But since the dissolution of the Warsaw Pact and of the Russian Empire itself, NATO has struggled to seek new raisons d'etre, and has succeeded to some degree in finding them in crisis management in Europe and in areas beyond the boundaries of the alliance.