military alliances in the classical realist tradition of E. H. Carr, Hans Morgenthau, George Kennan and Reinhold Niebuhr. Neorealism is subdivided into defensive and offensive neorealism. During peace-time, according to a 2019 study, almost all alliances from 1870 to 1916 may have been covert or implied. In other time periods, covert alliances have been rare. The study argues that from 1870 to 1916, the unusual amount of covert alliances was incentivized by other covert alliances. The creation of public alliances would signal to the covert ally that the public alliance was more valuable. According to Ronald Krebs, pre-WWII alliances were generally "relatively simple, short-lived affairs." Common problems for alliances revolve around free-riding and burden-sharing. Members of an alliance have incentives not to contribute to the alliance while simultaneously benefiting on the public goods provided by the alliance. According to Mancur Olson and Richard Zeckhauser's classic study of alliances, small states frequently free-ride on the large state's contributions to an alliance. Small allies that are militarily vulnerable are less likely to free-ride, whereas strategically important small allies are most likely to free-ride. Alliances may also lead to moral hazard whereby allies behave more aggressively and recklessly if they believe that the alliance will aid them in any conflict. On the whole, alliances do deter aggression on net. Within alliances, actors may fear entrapment or abandonment. Entrapment means that allies get dragged into a conflict over one ally's interests that the other allies do not share. Abandonment means that allies do not come to the rescue of a fellow ally. Strong commitments to an alliance can reduce the bargaining power of that ally vis-a-vis the other allies. However, an ally whose commitment to the alliance is in doubt has greater bargaining leverage. Weak alliance commitments can make it easier for the ally to realign the alliance if a fellow ally is considered unsatisfactory. Strong alliance commitments may strengthen the adversary's alliance, as the adversary may face a greater threat. The failure of a strong ally to come to the rescue of a weaker ally (abandonment) may jeopardize the strong ally's other alliances. However, it may also strengthen the other alliances, as the other allies may sometimes prefer that the strong ally abandons a weak ally if it is likely to raise the risks of military escalation for the other allies. European historiography In the European historical context, a military alliance can be