There is not a U.S. Japan, however the United States does have various armed forces stationed in Japan. The U.S. is obligated to defend Japan under the Treaty of Mutual Cooperation and Security between the United States and Japan.

metropolitan planning organizations

(the "A" may simply refer to a specific area or branch)

Japan has only one formal ally: the United States. The two countries have been allies since signing the Treaty of Mutual Cooperation and Security in 1960.

Germany, Italy and Japan signed a mutual defense treaty.

Nuclear test ban treaty, United States-Japan security treaty, and the Indus Waters Treaty