Korea, including the U.S. Eighth Army, Seventh Air Force, and U.S. Naval Forces Korea. The two nations have strong economic, diplomatic, and military ties, although they have at times disagreed with regard to policies towards North Korea, and with regard to some of South Korea's industrial activities that involve usage of rocket or nuclear technology. There had also been strong anti-American sentiment during certain periods, which has largely moderated in the modern day. The two nations also share a close economic relationship, with the U.S. being South Korea's second largest trading partner, receiving $66 billion in exports in 2016. In 2007, a free trade agreement known as the Republic of Korea-United States Free Trade Agreement (KORUS FTA) was signed between South Korea and the United States, but its formal implementation was repeatedly delayed, pending approval by the legislative bodies of the two countries. On 12 October 2011, the U.S. Congress passed the long-stalled trade agreement with South Korea. It went into effect on 15 March 2012. Military Unresolved tension with North Korea has prompted South Korea to allocate 2.6% of its GDP and 15% of all government spending to its military (Government share of GDP: 14.967%), while maintaining compulsory conscription for men. Consequently, South Korea has the world's seventh largest number of active troops (599,000 in 2018), the world's highest number of reserve troops (3,100,000 in 2018). The South Korean military consists of the Army (ROKA), the Navy (ROKN), the Air Force (ROKAF), and the Marine Corps (ROKMC), and reserve forces. Many of these forces are concentrated near the Korean Demilitarized Zone. All South Korean males are constitutionally required to serve in the military, typically 18 months. Previous exceptions for South Korean citizens of mixed race no longer apply since 2011. In addition to male conscription in South Korea's sovereign military, 1,800 Korean males are selected every year to serve 18 months in the KATUSA Program to further augment the United States Forces Korea (USFK). In 2010, South Korea was spending ₩1.68 trillion in a cost-sharing agreement with the US to provide budgetary support to the US forces in Korea, on top of the ₩29.6 trillion budget for its own military. The South Korean Army has 2,500 tanks in operation, including the K1A1 and K2 Black Panther, which form the backbone of the South Korean army's mechanized armor and infantry forces. A sizable arsenal of many artillery systems, including 1,700