Last week one of the U.S. Navy's newest patrol aircraft, a P-8A Poseidon, took off from an air base in the Philippines and flew due west.

Over the shimmering blue green waters of the South China Sea, the plane was hailed by a voice warning the crew they were nearing Chinese territory and should immediately leave.

The incident didn't occur near what most people consider China — mainland China was hundreds of miles away. It occurred near three tiny islets in the South China Sea: Subi Reef, Fiery Cross Reef, and Mischief Reef. These tiny, seemingly inconsequential bits of land are the front lines in a dispute involving China, her neighbors, and now the United States.

The islets could also be stepping stones in the path to war, as China and the United States become increasingly embroiled in a territorial dispute that neither side has any intention of backing down from.

The South China Sea is one of the most valuable patches of ocean in the world. A third of the world's merchant traffic passes through the area. It's also resource rich, home to rich fishing grounds and large reserves of oil and natural gas. The South China Sea functions as a sea border for a number of countries, including China, Vietnam, Malaysia, Brunei, Indonesia, the Philippines, and Taiwan.

In recent years, China has laid claim to roughly 90 percent of the South China Sea. This runs roughshod over territorial claims by China's neighbors. Furthermore, over the last several months, China has transformed Subi, Fiery Cross, and Mischief Reefs into outposts of the People's Republic, using dredgers to create land for building port facilities, radar stations, and airports.

China has agreed to abide by international regulations regarding the oceans. That sounds reassuring, but apparently the fine print is that it doesn't apply to Chinese territory. As far as China is concerned, international law applies to the South China Sea no more than it does to San Francisco Bay or Lake Michigan.

The U.S. Navy P-8A Poseidon was flying what the U.S. military calls a "Freedom of Navigation" mission, demonstrating that the United States does not recognize China's claims and that anyone — including the U.S. military — can transit through the area unmolested.

This week a Chinese military spokesman accused the United States of preparing to confront China over the issues of ownership of the South China Sea. The spokesperson, Senior Colonel Yang Yujun, claimed that the U.S. was "smearing the Chinese Navy," presumably for no other reason than it is jealous of China's rise.

Senior Colonel Yang is right but for the wrong reasons. The United States is preparing to confront China. The purpose isn't to smear the Chinese Navy, but rather to uphold a key strategic principle.

As a direct interest, the South China Sea isn't of huge importance to America. The principle of freedom of navigation, on the other hand is very much an interest of America's. As a maritime power surrounded by two oceans and reliant on ocean-going trade, the United States must on principle resist all such claims — even those by countries as small as the Maldives.

The United States is the only country capable of countering China's claim to the South China Sea. Despite the extraordinary growth and progress of the Chinese military over the past two decades, the U.S. military is still more powerful by a wide margin.

China is currently operating from a position of weakness. That's precisely why it must be countered now, before it grows more powerful. It's important to confront China over this issue now and — to borrow a term Chinese officials often use — "teach it a lesson."

Left unchecked, we don't know where China's ultimate ambitions might lie. It's best we don't find out.

In international relations, little is cut and dried. Complexity rules. The various, competing claims to the South China Sea make for a complicated situation, each country alternately proposing and disposing differing territorial boundaries.

One thing is cut and dried: China — or any other country — cannot be allowed to take the South China Sea unilaterally. Doing so would let might make right, and embolden the country to make more claims in the future. This will make for some tense moments above, on, and below the South China Sea, and could even conceivably start a war.

But simply letting China do whatever it wants is not an option. The risks are much smaller now, while China is weak, and infinitely better than the alternative of confronting a stronger, more powerful China down the road. The sooner China learns to play by the rules, the better.