A military alliance is a formal agreement between nations concerning national security. Nations in a military alliance agree to active participation and contribution to the defense of others in the alliance in the event of a crisis. In the event a nation is attacked, members of the alliance are often obligated to come to their defense regardless if attacked directly. In the aftermath of the Second World War military alliances usually behave less aggressively and act more as a deterrent. Military alliances can be classified into defense pacts, non-aggression pacts, and ententes. Alliances may be covert (as was common from 1870 to 1916) 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. Characteristics Military alliances are related to collective security systems but can differ in nature. An early 1950s memorandum from the United States Department of State explained the difference by noting that historically, alliances "were designed to advance the respective nationalistic interests of the parties, and provided for joint military action if one of the parties in pursuit of such objectives became involved in war." A collective security arrangement "is directed against no one; it is directed solely against aggression. It seeks not to influence any shifting 'balance of power' but to strengthen the 'balance of principle.'" The obvious motivation in states engaging in military alliances is to protect themselves against threats from other countries. However, states have also entered into alliances to improve ties with a particular nation or to manage conflict with a particular nation. The nature of alliances, including their formation and cohesiveness (or lack thereof), is a subject of much academic study past and present, with the leading scholars generally considered to be Glenn Snyder and Stephen Walt. Other schools discussing military alliances in international relations include the neorealist school introduced in 1979. It was first outlined by Kenneth Waltz in his 1979 book Theory of International Politics. Alongside neoliberalism, neorealism is one of the two most influential contemporary approaches to the study of military alliances in international relations; the two perspectives dominated international relations theory from the 1960s to the 1990s. Neorealism emerged from the North American discipline of political science, and reformulates the understanding of