Article V of Treaty of Mutual Cooperation and Security between the United States and Japan says:
Each Party recognizes that an armed attack against either Party in the territories under the administration of Japan would be dangerous to its own peace and safety and declares that it would act to meet the common danger in accordance with its constitutional provisions and processes.
Let us examine three questions here.
First, are the Senkaku Islands considered to be “the territories under the administration of Japan”?
Yes, not only the government of Japan but also the U.S. government repeatedly confirmed it. But China has been recently challenging the administration of Japan by having the Chinese coast guards intrude the territorial waters off the Senkaku Islands. If Chinese public vessels try to enforce Chinese law against Chinese fishing boats in the related sea area, the Japanese government will consider that Beijing has completely cross a red line, and take some countermeasures.
Second, what is “an armed attack” under the Treaty?
If Chinese navies rush to the Senkaku Islands and clash with the Self Defense Forces, it will be no doubt an armed attack on Japan.
What if the Chinese coast guards clash with the Japan Coast Guards in the territorial waters off the Senkakus? Although the Chinese coast guards, including the Maritime Surveillance, are known to have close relationship with the PLA, they are regarded as the maritime policing agency, not the military organization. The JCG will cope with them in the first place. The type of clash could be interference of course, collision of vessels, shooting of warning shots and/or water cannon. Even these activities may incur casualties, but I guess it will be difficult to interpret them as an armed attack on Japan at these stages. If the Japan Coast Guards and the Maritime Surveillance exchange fire in harmful way and for a considerable time, the likelihood of Washington’s admitting the situation as an armed attack will increase, but still not be large enough.
The clash may not end here. If the situation escalates further, the confrontation between the SDF and the PLA cannot be ruled out. For Japanese side, the government will order “maritime security operation,” “public security operation,” or “defense mobilization” to the SDF. By Japanese law, the last one responds to an armed attack on Japan, and the use of force will be fully possible. At this stage, it will be difficult for the U.S. government not to admit it as an armed attack.
Third, what are the actions to “meet the common danger in accordance with its constitutional provisions and processes”?
As stated above, the Japanese government will mobilize the SDF for defense when the situation meets the legal requirement, including the Diet approval. The response of the U.S. government is not as clear. The views are divided whether President can start war without the approval of the Congress. But President Obama gave up bombing Syria in September 2013 as he failed to obtain Congressional approval. President will need Congressional support, at least politically, in order to send the U.S. forces for the defense of Japan. If the Congress and the U.S. public are hesitant to make war with China over the tiny—for most of the Americans—islands of the Senkakus, that there will be a possibility that President cannot decide a direct military intervention. It is also possible that President himself/herself may not to want to do so.
In the real world, political calculations always matter. And the language of the Japan-U.S. Security Treaty is vague enough compared to that of the North Atlantic Treaty. The U.S. government will have to weigh the cost and benefit of action against those of inaction. The decision to abandon Japan will cost the United States as much. To choose either Japan or China is an extremely difficult task for any U.S. president.