War in Asia may be unthinkable, but it is getting easier to imagine scenarios that could see China, Japan and conceivably the United States in a shooting match. China’s announced expansion of its restricted air zone over waters adjoining the disputed Senkaku/Diaoyu islands drew strong reactions from Japan — which rejected China’s claim — and from the United States.China says that planes wishing to fly into the zone (which include a significant area of international territory) must identify themselves or face unspecified “defensive emergency measures.” The United States immediately announced that it will ignore the zone and that US military flights off the Chinese coast will continue as before.Was China just blowing smoke or does it really mean what it says? We are likely to find out before long. Both Japan and the United States will be testing this zone in the days and weeks to come.Many Americans like to think that the peaceful and, from a US standpoint, basically benign world order we inhabit is a fact of nature, something that can be indefinitely sustained with no heavy lifting on our part. Nothing, unfortunately, could be farther from the truth, and at a time when the United States is sending confusing signals to the world about our will to do what it takes to uphold that world system, we must expect that various countries are going to test us from time to time to see whether we are still committed.China is one of the countries most interested in testing American commitments, but countries like Japan are equally concerned. American strength and American commitment are indispensable to stability in Asia; American politicians need to weigh their words and their actions carefully. Asia is watching.