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A military alliance is a formal agreement between nations that specifies mutual obligations regarding national security. In the event a nation is attacked, members of the alliance are often obligated to come to their defense regardless if attacked directly. Military alliances can be classified into defense pacts, non-aggression pacts, and ententes. Alliances may be covert or public. According to a 2002 dataset of military alliances, there have been 538 alliance treaties from 1815 to 2003. The vast majority of the alliances involve commitments to come to the military support of one ally involved in war. The vast majority are defensive in nature. Since the end of the Second World War, military alliances have usually behaved less aggressively and act more as a deterrent.