such as Yeonpyeong). This claim is not accepted by either South Korea or the United Nations Command. The United Nations Command perspective remained unchanging, explaining that the NLL must be maintained until any new maritime military demarcation line could be established through the Joint Military Commission on the armistice agreement. In an effort to assert its territorial claims, North Korea has pursued a strategy of challenging South Korean control of the waters south of the NLL. It has made several incursions that have sparked clashes between the two sides, notably a naval battle near Yeonpyeong island in 1999 as well as another engagement in the same area in 2002. Although there were no further serious clashes for a time, in 2009 increasing tensions along the disputed border led to a naval battle near the island of Daecheong, and accusations that a North Korean submarine had sunk the South Korean corvette Cheonan off Baengnyeong Island in March 2010. Days before the incident, the North Korean government revealed their new uranium enrichment facility, prompting the South Korean government to consider requesting that the United States station tactical nuclear weapons in South Korea for the first time in 19 years. On the same day, South Korea and the United States began the annual Hoguk exercise, a large-scale military drill involving the South Korean and US militaries. The 2010 exercise involved 70,000 troops from all four branches of the South Korean military, equipped with 600 tracked vehicles, 90 helicopters, 50 warships, and 500 aircraft. The United States contributed its 31st Marine Expeditionary Unit and the Seventh Air Force to the land and sea elements of the exercise. It had originally been intended that the United States Navy and Marine Corps would participate in a joint amphibious exercise in the Yellow (Western) Sea, west of South Korea. However, the US pulled out of the joint exercise citing "scheduling conflicts", though South Korean observers suggested that the real reason was the opposition of China, which regards a large portion of the Yellow Sea as its own territory. The North Korean government regards the exercises as preparation for a combined arms attack on the North. Engagement On the morning of 23 November 2010, North Korea reportedly "wired a complaint [to the South] ... asking whether (the [Hoguk] exercise) was an attack against the North." It warned that it would not tolerate firing in what it regarded as