Prime Minister Koizumi Junichiro has done an extraordinary job in leading Japan’s response to the Sept. 11 terrorist attacks on the United States. Both the package of measures his government put together to support the U.S.-led coalition against terrorism and the speed with which it was approved have been nothing less than astounding. But the prime minister’s work is not yet done. The dispatch of Self-Defense Forces (SDF) has renewed questions about Japanese intentions and ambitions. Mr. Koizumi must respond to those concerns: He has an historic opportunity to build a new framework for Japan’s relations with its neighbors.