The United States Air Force has sent an air contingent to the Philippines amid escalating tensions between the island nation and China. The tension is over a territorial dispute involving the Spratly Islands; a chain of islands thought to have great economic value.

The force sent included 200 airmen along with A-10 Warthogs from the 51st Fighter Wing and helicopters from the 18th Air Wing based in Japan. Another thirty airmen are expected to join them soon from Hawaii. The Philippines, Australia, and the United States recently conducted annual military exercises with each other, involving over 8,000 service members.

This comes after United States Secretary of Defense Ash Carter and Philippine Defense Secretary Voltaire Gazmin declared an “enhanced military alliance” between the Philippines and the United States. The United States and the Philippines signed the Enhanced Defense Cooperation Agreement back in 2014, which allowed for the United States to build and operate facilities on Philippine military bases.

Tensions between the Philippines and China have been rising since 2012 when the Chinese sent fishing vessels to Scarborough Shoal, which the Philippines claims it has control due to its falling within its Economic Exclusive Zone (EEZ.) The Philippines and China have been waging a war of words for the past three years.

China lays claim to the South China Sea where a massive amount of trade passes through each year. It is also thought to have great amounts of oil reserves. China has a nine dashed line, where it claims its territories. Many nations, including the Philippines, Taiwan, Vietnam, and Brunei dispute the nine dash line.

The Philippines has responded by increasing its military spending and modernization. For decades the Philippines has been focused on internal affairs, waging a war with Moro rebels in Mindanao and communist rebels elsewhere. Now the Philippines has been forced to focus on external defense, aiming for what call a “minimum credible defense.” The Philippines has purchased anti-submarine helicopters, fighter aircraft, and other military equipment. They have also filed an arbitration case against China, which has refused to participate in the arbitration. The international tribunal in the Hague has declared it has jurisdiction over the case.

The Philippines has been pursuing an alliance with Vietnam, with the two countries exploring joint military patrols this month in the South China Sea. The two nations signed a defense agreement in 2010. Vietnam has also supported the Philippines arbitration case against China, rejecting China’s so-called nine dash line.

The United States has been supporting the Philippines, promising to respect the court’s ruling (when they make one). The United States has also sailed warships within twelve miles of Chinese islands in the South China Sea, the most recent in January. The United States and the Philippines began conducting joint patrols in the South China Sea this month.

The United States has been transferring military equipment to the Philippines, as well. Two former Coast Guard cutters have become the main ships of the aging Philippine Navy, with a third expected to be transferred in the near future.

The State Department, back in 2014, released a report saying that China’s nine dash line was not within international law of the sea. A U.S senior official also remarked that China had to legally abide by the tribunal’s decision due to both the Philippines and China being signatories of the United Nations Convention on the Law of the Sea.

Adding to the tension, China lately has decided to build floating nuclear power plants in the South China Sea according to the Washington Times.