In January 1960 the US and Japan signed a treaty of mutual cooperation and security that formalized a US-Japanese military alliance.
Left-wing protests roiled the government of Nobosuke Kishi as he pushed through the revised security treaty with America.
The treaty permitted the presence of U.S. military bases on Japanese soil, and committed the two nations to defend each other if one or the other were attacked in the territories under the administration of Japan.
Source: Wikipedia