While most of the world has its attention on the Korean peninsula, tensions continue to rise between China and the United States in another part of the Pacific region as well: the South China Sea.

On Wednesday, a U.S. Navy destroyer sailed near territory that China has claimed sovereignty over in another of a series of Freedom of Navigation missions intended as a sort of diplomatic show of force, making it clear to the Chinese government that the United States does not acknowledge China’s ownership of what now constitutes nearly the entire waterway.

What the U.S. vessel did violated China’s sovereignty and security interests, put the safety of Chinese vessels and personnel who were in the relevant waters for official duties under grave threat, and contravened the basic norms for international relations,” Foreign Ministry Spokesman Lu Kang said. “China is strongly dissatisfied with that and will take necessary measures to firmly safeguard its sovereignty.”

The South China Sea is among the most heavily-trafficked commercial shipping routes on the globe, seeing nearly a third of all commerce at one time or another. Further, it is said to contain significant natural resources, including oil and large fish populations. As a result, China has expanded its claims over the territory from the traditional 12 nautical miles out hundreds of miles, to areas that even include seas just off the coast of other nations.

This map shows China’s growing claim over the South China Sea in red, with overlapping claims of other nations shown in other colors. (Wikimedia Commons)

Wednesday’s Freedom of Navigation mission took the USS Hopper, a U.S. missile destroyer, to the very edge of that international norm, moving to just about exactly 12 nautical miles from China’s claimed Huangyan Island. China responded by threatening to take “necessary measures” to ensure protection of its sovereignty, an ironic statement, as that particular island is also claimed by the Philippines.

“All operations are conducted in accordance with international law and demonstrate that the United States will fly, sail, and operate wherever international law allows,” Pentagon spokesman Lieutenant Colonel Christopher Logan said of the passage.

Another official, speaking on the condition of anonymity, claimed that the USS Hopper may have even violated the 12 nautical mile rule, but did so in accordance with international norms for what is referred to as an “innocent passage.” That phrase usually is used to describe a ship traversing claimed waterways in as direct and quick a manner as possible out of respect for the claim.

China’s rapidly expanding military is aided in the defense of this newly claimed territory through President Xi’s widespread re-organization and modernization efforts for China’s People’s Liberation Army. The nation has also been developing man-made islands for placing fortifications and defensive assets, seemingly demonstrating a willingness to engage other nation’s militaries in order to defend what is effectively an annexation of international waters.