According to Article 9 of the Japanese Constitution:
(1) Aspiring sincerely to an international peace based on justice and order, the Japanese people forever renounce war as a sovereign right of the nation and the threat or use of force as means of settling international disputes.
(2) In order to accomplish the aim of the preceding paragraph, land, sea, and air forces, as well as other war potential, will never be maintained. The right of belligerency of the state (right to wage war) will not be recognized.
Because of this article in the constitution, Japan had only the U.S. occupation forces and a minor domestic police force on which to rely for security after the war ended in 1945. The Japan Self Defense Force, or the JSDF, was formed in 1954 due to rising Cold War tensions and the Korean War. There are three branches to the JSDF: the Japan Ground Self Defense Force (Army); the Japan Maritime Self Defense Force (Navy); and the Japan Air Self Defense Force (Air Force).
In the years after World War Two (1939-1945), Japan's relations with the United States were placed on an equal footing beginning with the end of the occupation by the Allied forces in April 1952. This equality, the legal basis of which was decided in a peace treaty signed by forty-eight Allied nations and Japan, was initially largely nominal. Under the terms of that treaty, the Treaty of Mutual Cooperation and Security, United States forces stationed in Japan were to deal with external aggression against Japan while Japanese forces, the JSDF, would deal with internal threats and natural disasters.
After World War Two, the Japanese worked to recover stability within their country. Self-confidence grew as the country applied its resources and organizational skill to regaining economic health. This situation gave rise to a desire for greater independence from the influence of the United States. During the 1950s and 1960s, this feeling was especially evident in the Japanese attitude toward United States military bases.
The Japan Self-Defense Forces are controlled by the Ministry of Defense. In recent years they have been engaged in international peacekeeping operations, including UN peacekeeping missions. In 2015, the Diet enacted legislation that allowed the JSDF to defend allies, as well. Recent tensions, particularly with North Korea, have reignited the debate over the status of the JSDF and its relation to Japanese society.
The Clash: US-Japanese Relations Throughout History by Walter LaFeber, W W Norton & Co Inc; New York, 1997.