the United States Taiwan Defense Command. Nuclear weapons are known to have been stored at Tainan Air Force Base until their withdrawal was ordered by the American President in 1972. High-level cooperation ended with the US recognition of the PRC in 1979, when all remaining US forces in Taiwan were withdrawn. The US continued to supply the ROC with arms sales per the Taiwan Relations Act, albeit in a diminished role. When the United States Congress enacted on September 30, 2002, the Foreign Relations Authorization Act for FY 2003, it required that Taiwan be "treated as though it were designated a major non-NATO ally." Despite some initial misgivings about Congress's perceived intrusion into the President's foreign affairs authority, the Bush administration subsequently submitted a letter to Congress on August 29, 2003, designating Taiwan as a major non-NATO ally. In recent years, the ROC military has again begun higher level cooperation with the U.S. Military after over two decades of relative isolation. Senior officers from the U.S. Pacific Command observed the annual Han Kuang military exercises in 2005. The US also upgraded its military liaison position in Taipei from a position held by retired officers hired on a contractual basis to one held by an active duty officer the same year. The United States regularly sends personnel to Taiwan for both training and liaison purposes but does so either secretly or in an unofficial capacity. ROC Marines have trained with their American counterparts in Hawaii and US Marines have also deployed to Taiwan. In 2015 two United States Marine Corps F/A-18C Hornets made an unscheduled landing at Tainan Air Force Base after one of them developed an engine anomaly in-flight. The aircraft were accommodated in an air force hangar until a C-130 full of American technicians could be flown in to check them out. Tsai Ing-wen's request of purchasing weaponry from the US was approved by the US State Department in July 2019. The deal includes 108 Abrams tanks, 250 Stinger missiles and related equipment worth $2.2 billion. Tsai said the weaponry would "greatly enhance our land and air capabilities, strengthen military morale and show to the world the US commitment to Taiwan's defense." In May 2020, the U.S. Department of State approved a Foreign Military Sale of 18 MK-48 Mod 6 Advanced Technology Heavy Weight Torpedoes for Taiwan in a deal estimated to cost $180 million. Elite units of the