The membership of the organization was then largely static. In 1974, as a consequence of the Turkish invasion of Cyprus, Greece withdrew its forces from NATO's military command structure but, with Turkish co-operation, was readmitted in 1980. The Falklands War between the United Kingdom and Argentina did not result in NATO involvement because Article 6 of the North Atlantic Treaty specifies that collective self-defense is applicable only to attacks on member state territories north of the Tropic of Cancer. On 30 May 1982, NATO gained a new member when the newly-democratic Spain joined the alliance, as was confirmed by referendum in 1986. At the peak of the Cold War, 16 member nations maintained an approximate strength of 5,252,800 active militaries, including as many as 435,000 forward-deployed US forces, under a command structure that reached a peak of 78 headquarters, organized into four echelons.