On July 21, 1949, the United States ratified the North Atlantic Treaty. The North Atlantic Treaty created the North Atlantic Treaty Organization (NATO).

The most important section of the treaty was Article V, which committed each member state to consider an armed attack against one state to be an armed attack against all states. The United States ratified the North Atlantic Treaty on July 21, 1949. Twelve nations originally ratified the treaty, including: Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom, and the United States. The members signed the Treaty in Washington, D.C.

NATO is an intergovernmental military alliance based on the North Atlantic Treaty. The organization constitutes a system of collective defense, meaning that its member states agree to defend each other in response to an attack by an external party (i.e., a state which is not a member).

For its first few years, NATO was not much more than a political association between nations. The Korean War, however, changed that. Member states built an integrated military structure to participate in the war, thus beginning NATO’s participation in international war and politics. Upon last count, NATO has 28 member states.