FEW VISITORS to Washington have made a more extensive effort than Japan's prime minister, Yasuhiro Nakasone, to earn a warm welcome. While all Japanese leaders have understood and accepted the centrality of the tie with the United States, Mr. Nakasone has shown a rare directness and political courage in addressing the current requirements of the relationship. In office barely two months, he has thrown himself into the task of assessing, announcing and correcting some of its major weaknesses.

Before the new year, Mr. Nakasone had taken certain steps in the key areas of trade and defense. Even those Americans, including ourselves, who felt that Japan had long neglected its obligations in these areas, and that the new steps did not go far enough, were prepared to acknowledge that Mr. Nakasone was pushing hard against real political constraints in his country. Since then, his government has been at pains to point out that it is continuing its efforts to build political support for further efforts both to open Japanese markets wider to foreign products and to take on a share of its defense more commensurate with its capacities and needs. The prime minister is talking about these matters to the Japanese in the spirit of a leader who accepts that he must tell hard facts to his public in order to bring it along a difficult road. In recent days, Mr. Nakasone has found it possible to move in several policy areas where previous Japanese leaders had felt comfortable with stalemate and inaction. Through the 32 years of Japanese-American security ties, for instance, the transfer of military technology has been strictly a one-way affair--from the United States, notwithstanding Japan's increasing capabilities. The Japanese trade and industrial establishment blocked any sharing of Japan's military know-how. Mr. Nakasone has now approved the principle of sharing. He has done it, moreover, not as the result of the usual tedious, painful, years-long negotiation; he has simply done it.

Mr. Nakasone also has broken an internal logjam regarding South Korea. Last week, he made the first trip to Seoul a Japanese prime minister has ever taken for official talks, taking with him major economic aid credits. Its history and policy keep Japan from setting up a direct security link with South Korea, though both are American allies. But Mr. Nakasone did accept a major new responsibility for underwriting the security of an important neighbor by non-military means. The United States had long encouraged Japan's move in this direction, and Mr. Nakasone has done it.