Yasuhiro Nakasone, visiting Washington last week for the first time as Japan's prime minister, turned out to be that unusual leader with a coherent vision and considerable political courage. Just as Japan's unchecked military expansion brought it isolation and ultimate disaster in World War II, he pointed out, so now the political leadership must ensure that Japan's economic expansion does not produce a contemporary isolation. He pledged to help Japan become "a harmonious member of the international community. . . ."

Americans will rightly look first for the fruits of Mr. Nakasone's vision in trade. Japan's vigorous pursuit of American markets and its stinginess in opening its own have corroded relations. The prime minister had taken certain symbolic steps before arriving. Here he indicated he understands the urgency of the problem and intends to deal with it, but must proceed carefully, one step at a time. Most Americans he met seem to have smiled through gritted teeth and said they appreciated his political constraints but expect him to deliver.

In defense, Mr. Nakasone came through handsomely. He did so, we suspect, in part to make up for the stickier movement on trade and in part to adjust to Soviet realities: Moscow refuses to follow the American example and return Japanese territory taken in World War II, and it has substantially strengthened its forces threatening Japan. For years, Washington has urged Tokyo to take a broader view of the requirements of "self-defense." Mr. Nakasone did, accepting as missions to set up an air defense against Soviet Backfire bombers, to be able to keep Soviet subs and surface ships from transiting Japan's straits in an emergency, and to extend the protection of Japan's sea lanes.

The Kremlin reacted quickly and crudely, addressing specifically Mr. Nakasone's intent to stop intruding Soviet bombers--an entirely defensive mission--and raising the prospect of a nuclear strike on Japan. Even before the Nakasone visit, however, Yuri Andropov was reported to have threatened to redeploy SS20 missiles against Japan. Bullying is not a new Soviet tactic.

Mr. Nakasone's defense remarks also launched a hot debate in Japan that will surely test to the utmost his political skills. He was bold on his trip here. But we think he did no more than recognize that Japan lives in an unpleasant and demanding world. He went beyond the pieties that customarily cloak Japanese-American relations, and he seems quite prepared to cope with the consequences.