NATO, the North Atlantic Treaty Organization, is a military alliance established in 1949 by the North Atlantic Treaty for purposes of collective security. Article 5 of the treaty states that an attack on one member is considered an attack on all, obligating other members to assist. The only time Article 5 has been invoked was by the United States after the September 11 attacks. Donald Trump has repeatedly questioned the financial contributions of NATO members. - Donald Trump has cast doubt on the US commitment to defend NATO allies who do not meet defense spending targets. - Trump's statements are raising concerns in Europe and Asia about the reliability of US security support. - Trump also expressed doubt about NATO's commitment to defend the U.S. - NATO invoked its defense guarantee (Article 5) after the September 11 attacks to support the United States. - Trump has repeatedly questioned the financial contributions of NATO members.