What will the United States do if the Japanese and Chinese coast guards or the two militaries clash over the Senkaku Islands?

Needless to say, the United States and Japan are the allies based on Japan-U.S. Security Treaty. The U.S. forces have been stationing in Japan for more than seven decades. The United States has strategic concern over rising China as Beijing has seemingly started challenging the present word order. The U.S. trade in goods with Japan in 2015 amounted to 194 billion dollars. And the United States shares the value of democracy, freedom, human rights, and the rule of law with Japan much more than with China. Then, can we expect the United States to militarily intervene to help Japan? The answer is not simple.

For the United States, China is at least as important as Japan. The U.S. trade in goods with China in 2015 reached 599 billion dollars, tippling that with Japan. Washington needs Beijing’s cooperation in controlling North Korea and other bad guys. China has a veto power as a permanent member of the U.N. Security Council. Despite the Pivot to Asia strategy, Obama administration has had no choice but to continue to give priority on Middle East and/or Russia. Furthermore, Chinese military is too strong to make war with. Even if the U.S. military can finally win—I believe it will—, the casualties and costs will not be comparable to the war against Saddam Hussein’s Iraq or Taliban.

Officially, the U.S. government has an established view on the Senkaku Islands. That is, the U.S. government admits that article 5 of the U.S.-Japan Security Treaty covers all the territories under the administration of Japan, including the Senkaku Islands, while refusing to take a position on final sovereignty on the Senkakus. This basic line was made clear by early 2000s at the latest, after Ambassador Walter Mondale made a confusing comment in 1996 on the applicability of the Security Treaty in case of a possible Japan-China conflict over the Senkakus Islands. The more China increased provocation around the Senkakus, the higher ranking U.S. government officials, including President Barak Obama in April 2014, repeated this line. Recently, the United States added that the Senkakus have been historically administered by Japan and its sovereignty should not be subject to change unilaterally.

The United States will continue to say that the Security Treaty covers the Senkaku Islands in the foreseeable future. Although I am not sure about President Donald Trump, the U.S. has at least four reasons to do so. First is the historical involvement. The United States administered the Senkaku Islands after the World War II, and returned it to Japan in May 1972. Second, if Washington says that the Treaty does not cover the Senkakus, Japan will see little value on the alliance. The United States will lose the most precious ally in Asia with sophisticated military and big economy, as well as the U.S. bases in Japan. Third, if Washington takes ambiguous position on the applicability of the Treaty over the Senkakus, other U.S. allies would think the United States recoils from China. Then the credibility of the U.S. alliance all over the world will be severely damaged. And finally, the change of the current position may send a devastating signal to Beijing that Washington would tolerate China’s taking over the Senkaku Islands. In January 1950, Secretary of State Dean Acheson failed to mention Korean Peninsula when he made a speech about the lines of defense, which led Kim Il-Sung to expect that the United States would not intervene even if North Korean military invades south.

Japan has appreciated the U.S. commitment to apply the Security obligation over the Senkakus. But how effective was it to actually deter Chinese provocation? The answer is mixed. On the one hand, Beijing has so far advanced the scope of activities gradually in the East China Sea, without invoking the Treaty obligation for the United States. On the other hand, China has never tried to use force and seemingly avoided material clash with Japan in the sea area around the Senkaku Islands.

Will the announcement of the U.S. Treaty commitment over the Senkakus continue to deter China’s use of force? As I already explained, there are cases where China finally uses force while intending not to. For example, the possibility of a clash between the two coast guards is higher than even, and it may escalate to a military conflict between the SDF and the PLA in the worst scenarios.

Will the United States be obliged to make war with China in order to defend Japan and the Senkakus, then? I would like to say yes from the standpoint of Japan. But objectively speaking, the interpretation of the Japan-U.S. Security Treaty will be heavily influenced by the political calculation of the U.S. government.