The article discusses NATO military activities carried out since the end of the Cold War. The great majority of these activities were conducted not under the provisions of the Washington Treaty, and in particular article 5, but rather on the basis of ad hoc decisions adopted by the North Atlantic Council in accordance with the Alliance strategic doctrines. These developments did not amount to a tacit revision of the Washington Treaty, nor rendered indispensable such a revision, although ad hoc decisions could be considered as international accords concluded in simplified form. Different legal grounds were invoked to justify – not always convincingly – these activities, which consisted of peacekeeping operations, implementation of peace accords, and military coercive measures. The most striking feature of NATO involvement in the management of international crises, in any event, remains the progressive erosion of UN Security Council authority, which culminated with the intervention in Kosovo. The recent crisis in Iraq, however, demonstrates that there is no agreement among NATO members on whether obtaining an authorization from the Security Council before resorting to force is a legal requirement or a matter of political expediency. Attention will finally be paid to the activities recently carried out under article 5 of the Washington Treaty, and in particular the naval operations conducted on the Mediterranean Sea and the measures taken to protect Turkey in the context of the United States led military campaign against Iraq.