ATO, the American Treaty Organisation, is a mutual defense pact headed by the United States. Headquartered in Mexico City, Mexico, the organization constitutes a system of collective defence whereby its member states agree to mutual defence in response to an attack by any external party. It covers most of North and South America (only Argentina, Honduras, Suriname, French Guyana, Chile are not in ATO). ATO hit upon rough ground in 1993, when Venezuela was admitted. While it was not a member, Brazil had some bad blood with ATO and they were very influential in the area. This led to ATO being divided against itself and Brazil being attacked by Argentina, since they assumed ATO would not be a unified front.