Operation Active Endeavour was a maritime operation of the North Atlantic Treaty Organization. It operated in the Mediterranean Sea and was designed to prevent the movement of terrorists or weapons of mass destruction. It had collateral benefits in enhanced security of shipping in general. It was one of the first military actions taken by NATO in response to an invocation of Article 5 of the North Atlantic Treaty which provides for collective defense and the first-ever operation conducted by the Alliance in direct application of the defense clause of the Treaty. In November 2016 it was replaced by the non-Article-5 Operation Sea Guardian. History The operation began on 4 October 2001 as one of the eight NATO responses to the 11 September attacks, although it did not formally begin until 16 October. The naval assets of Standing Naval Force Mediterranean (STANAVFORMED), which were participating in Exercise Destined Glory 2001 off the southern coast of Spain, were reassigned in order to provide an immediate NATO military presence in the Eastern Mediterranean. Prior to its cessation, the operation was conducted by a number of NATO military assets, including the Standing NATO Maritime Group 1 (SNMG1) and Standing NATO Maritime Group 2 (SNMG2). On 4 February 2003, the North Atlantic Council (NAC) decided to extend Operation Active Endeavour to include escorting non-military ships traveling through the Strait of Gibraltar to maintain security in the area and to secure the safe transit of designated Allied ships. On 29 April 2003, Task Force Endeavour began boarding operations following a NAC decision to enhance the effectiveness of the current naval operation against suspected terrorist activities in the Mediterranean. The boarding operations were conducted in accordance with the rules of international law and were of a compliant nature. Over 160 vessels had been boarded as of 1 June 2010. Several submarines of the Royal Norwegian Navy Ula class have been deployed in the Mediterranean Sea in support of the NATO Operation Active Endeavour, where their intelligence-gathering abilities have surpassed expectations. Their operational availability proved to be the highest of all the ships taking part in the operation. On 15 September 2006, NATO authorized the Russian ship Pytliviy to participate in Operation Active Endeavour. Since its inception, the ships of Active Endeavour have monitored over 100,000 vessels (as of June 2010) and conducted voluntary boardings of over 100. They have also escorted over 480 vessels through the Strait