In early August, Japan’s coast guard witnessed an unconventional Chinese assault on its territorial waters.

According to Japanese officials I met with last week, at least 300 Chinese “fishing vessels” began incursions into the exclusive economic zone around the uninhabited Senkaku Islands, disputed territory administered by Japan but claimed by China and Taiwan as well.

Japan has seen similar probing activities for years. But in August, the Chinese escalated. There were far more boats than before, and the Chinese sent armed coast guard vessels to accompany these “fishermen.”

This may sound fairly benign compared to the shooting wars in Ukraine and Syria. But for Japan, the matter could not be more serious. Its military assesses that many of these sailors are really Chinese militia irregulars, similar to the non-uniformed “little green men” whom Russia has sent to eastern Ukraine to stir up separatist sentiment. Call them “little green boats.”

Nonetheless, Japan has gone out of its way not to take China’s bait and respond militarily. Instead, it sends its own coast guard to escort the boats and inform them of their trespassing over loudspeakers.

Diplomatically, Japanese officials have taken to lodging angry, formal complaints in late-night phone calls to their Chinese counterparts, a tactic usually favored by Beijing’s envoys.

There are three reasons Americans should watch the rising tensions in the East China Sea carefully.

To start, President Obama in 2014 said publicly the United States was bound by its treaty with Japan to come to Japan’s aid if the Senkakus were ever attacked. In this sense, the islands have the same status as the Baltic states in the NATO alliance.

Unlike the red line in Syria that Obama ended up backing away from in 2013, the United States would be violating a formal agreement if it did not come to the defense of the Senkaku Islands.

Second, the Chinese maneuvers around what they call the Diaoyu Islands appear to be part of China’s larger strategy to gain control of the shipping lanes of the East China Sea, similar to its construction of artificial islands in the adjacent South China Sea.

Since the naval officer and historian Alfred Thayer Mahan wrote his treatise “The Influence of Sea Power Upon History” in 1890, the United States has considered the defense of free navigation on the high seas to be a pillar of its national security policies.

Finally, the Chinese activity near the Senkakus is another example of the rise of ambiguous warfare, where a state uses irregular forces or nonmilitary means to advance its territorial goals.

Kunihiko Miyake, president of the Tokyo-based Foreign Policy Institute for Global Studies, told me last week that China was trying to “win the islands without fighting.”

He added: “They know if they fought us, this would be a fight with us and the United States. That’s why they will not physically challenge us. They will use fishing boats with their unofficial maritime militias, but they don’t send military vessels yet.”

Evelyn Farkas, a former deputy assistant secretary of defense who was in my delegation of journalists and policy experts last week in Japan, told me, “What the Chinese military is doing in the South China Sea and the East China Sea resembles in some ways the approach of the Russian military in its incrementalism, in its ambiguity about whether it’s civilian or military, and in its challenge to the international community.”

This development ought to worry the current and next administrations. When Russia’s little green men swiftly took control of Crimea in 2014, it caught many in the Obama administration off guard. The Japanese are warning the United States not to make the same mistake with China’s little green boats.

Eli Lake is a Bloomberg View columnist who writes about politics and foreign affairs.

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