The terrorist attacks of Sept. 11 have changed the diplomatic momentum in Northeast Asia and Japanese Prime Minister Koizumi Junichiro has been the chief beneficiary of the new dynamic in the region. The calls for action on behalf of its American ally have given Japan and the Koizumi government the cover they need to make controversial decisions on security policies. To his credit, the prime minister has responded well: his desire to act as a good ally to the United States has been matched by unprecedented diplomatic efforts to allay the concerns of Japan’s neighbors. Now he must deliver on both fronts. Japan must follow through on the prime minister’s promises and match his good intentions with concrete acts. There are many in the region who do not expect Mr. Koizumi to deliver.