The North Atlantic Treaty Organization, also called the North Atlantic Alliance, and most commonly referred to by the acronym NATO, is an intergovernmental military alliance based on the North Atlantic Treaty which was signed on 4 April 1949. The organization constitutes a system of collective defence whereby its member states agree to mutual defense in response to an attack by any external party. NATO's headquarters are located in Haren, Brussels, Belgium, in which the always-American Supreme Allied Commander also resides. As of 2015, there are 28 member states across North America and Europe. An additional 22 countries participate in NATO's Partnership for Peace program, with 15 other countries involved in institutionalized dialogue programs. The combined military spending of all NATO members constitutes over 70 percent of the global total. Members' defense spending is supposed to amount to 2 percent of GDP.
Article 5 of the North Atlantic treaty, requiring member states to come to the aid of any member state subject to an armed attack, was invoked for the first and (thus far) only time after the September 11, 2001 attacks in the United States, after which troops were deployed to Afghanistan under the NATO-led ISAF.
While NATO continued to exist after the Yellowstone Supervolcano erupted in the United States, the consequences of the eruption proved to be a heavy blow to NATO overall. This was demonstrated six years after the eruption, when Russia, suffering from harsh winters and a complete failure of agriculture, invaded both Ukraine and Kazakhstan, which were somewhat better off. The Russians used the fact that both countries had historically been under Russian rule as a thin casus belli. Both countries reached out to NATO, but as most of NATO's members were also suffering ill-effects of the eruption, their response was limited at best. The U.S. Secretary of State expressed disapproval of the invasion. Russia effectively told the USA that the invasion was none of its business.