The United States maintains American military bases in Japan as part of the U.S.-Japan alliance of 1951. Most US military are in Okinawa Prefecture. In 2013 there were approximately 50,000 U.S. military personnel stationed in Japan with 40,000 dependents and 5,500 American civilians employed by the United States Department of Defense. The United States Seventh Fleet is based in Yokosuka, Kanagawa Prefecture. The 3rd Marine Expeditionary Force (III MEF) is based in Okinawa. 130 USAF fighters are stationed in the Misawa Air Base and Kadena Air Base. U.S. Fleet Activities Yokosuka is the largest and strategically most important U.S. naval base in the western Pacific. The base was previously the headquarters of the Yokosuka Naval District of the Imperial Japanese Navy, but now only a small portion of it is used by the JMSDF. Kadena Air Base is the largest and most active US Air Force base in the Far East. Japan pays 75% ($4.4 billion) of all US basing costs. Japan's willingness to host the majority of the US Armed Forces in Asia makes Japan essential to America's security policy in the Indo-Pacific. This helps the U.S. to project military force in the Pacific and Asia. The U.S.-Japan alliance is the cornerstone of peace, stability and economic prosperity in the Pacific.