The Trans-Atlantic Treaty Organization (TATO), also called the Trans-Atlantic Alliance, is an intergovernmental military alliance based on the Trans-Atlantic Treaty which was signed on 4 April 194?. 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. TATO's headquarters are in Copenhagen, Denmark, one of the 12 member states across Atlantis and Europa, the newest of which, Hungary and Serbia, joined in April 2009. 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. Most TATO nations are allied with NPTO nations as well, as the North Pacific Treaty and the Trans-Atlantic Treaty were both organized and led by the United States and Russia.
The course of the Cold War led to a rivalry with nations of the Amsterdam Pact, which formed in 1957. Doubts over the strength of the relationship between the Europan and the Atlantean ebbed and flowed, along with doubts over the credibility of the TATO defence against a prospective Britannic invasion. After the Big Thaw period, tensions TATO nations and Amsterdam Pact nations decreased significantly. Politically, the organization sought better relations with former Amsterdam Pact countries, several of which joined the alliance in 2001 and 2007.