When President Obama arrives in Tokyo on Friday, he will confront a country that seeks to be an ally of the United States. For Japan has never been an American ally. It was first a rival, then an enemy, and finally, after it lost the war it foolishly started with the U.S., it became a protectorate, not an ally. The distinction matters. An alliance is an institution negotiated between two sovereign governments in which each agrees to a series of reciprocal obligations that have the force of law.