A little more than 40 years ago the U.S. government made a fateful -- in retrospect historic -- decision not to interfere with the long-unbroken reign of the Japanese imperial family when American forces moved into Japan at the end of World War II. Yesterday Secretary of State George P. Shultz welcomed the next generation of the royal family and paid tribute to its role as "a rudder ... a mooring to basic values ... an anchor to weather the buffeting of changes" in one of the fastest-changing societies anywhere.

Shultz was toasting Crown Prince Akihito and Crown Princess Michiko at a State Department luncheon for 145 people shortly after the royal couple paid a call at the White House, where they met separately with President Reagan and First Lady Nancy Reagan. Akihito and Michiko were on the second day of their four-day stay in Washington, the second-to-last stop of their eight-day U.S. visit.

Akihito, 53, has spent about two-thirds of his life in preparation to be Japan's 125th emperor, succeeding his famous and long-serving father, Emperor Hirohito, who is 86. Akihito was officially invested as Crown Prince in 1952 and came to this country on his first visit the following year.

Shultz said in his toast to the royal couple that "Japan is blessed with an imperial family that not only symbolizes but stands for the nation's unique culture, unity, stability and continuity." He added, "Your country has shown the rest of the world how to draw strength and surety from tradition and at the same time move boldly forward."

Akihito referred to Commodore Perry's arrival in Tokyo Bay in 1853 and said that "today our relations are closer than ever, with the exchanges between our two people expanding throughout every field in an unprecedented manner."

The Japanese crown prince stayed away from controversies over trade and did not mention his government's new initiatives regarding military aircraft sales from the United States and aid to international efforts in the Persian Gulf. The Japanese royal family has strictly avoided political issues in the postwar era -- a policy that sharply limits what its leading figures can say, even on ceremonial occasions.

U.S. Ambassador to Japan Mike Mansfield, who was described by Akihito as enjoying "the respect and affection of every Japanese," took the unusual step of accompanying the imperial couple across the Pacific on their Japan Air Lines jet and at every step of their tour to Boston, Washington and New York. The couple was forced to cancel about half of their trip, which was also to include Charleston, S.C., Atlanta, Kansas City, Mo., Los Angeles, San Francisco and Hawaii, because of the emperor's recent intestinal surgery.

The 84-year-old Mansfield, who has spent 10 years as U.S. ambassador in Japan, was quoted by Akihito as saying that "the relationship between Japan and the United States is the most important bilateral relationship in the world, bar none." Mansfield explained later that he believes this because "I think that gradually we're becoming more dependent upon each other and because I know the next century will be the century of the Pacific." He said he felt that way since he first came to the Pacific as a U.S. Marine in the 1920s, long before his career as a U.S. senator from Montana and Senate Democratic Leader.

Among others in attendance at the State Department luncheon was Gaston J. Sigur, who had been a graduate student at the University of Michigan during the crown prince's 1953 tour and is now assistant secretary of state for East Asian and Pacific affairs. Sigur recalled that in 1953 he took Akihito to a practice session of the then top-rated Michigan football team, and that the Wolverines' quarterback tossed a long pass on the gridiron that the crown prince handily caught.

Akihito will have another chance to display his athletic prowess this afternoon when he is to meet Vice President Bush and Shultz on the tennis court at Bush's residence. The fourth member of the doubles match is to be tennis professional Pam Shriver. It could not be learned whether she will team up with the visiting royalty.