The Reagan administration has decided to call a truce in Washington's protracted diplomatic struggle for substantial Japanese boosts in defense spending. Officials in the United States and Japan remain at odds on how far and fast Tokyo should move to meet its military obligations.

Washington's outwardly conciliatory approach contrasts sharply with the get-tough tactics adopted by Carter administration officials to badger Tokyo into speeding up its yen-pinching military plans. It also reflects the current view among senior U.S. officials that politicians in Tokyo should be given more breathing room to prepare public opinion in Japan, where the question of an expanded defense role still raises strong antiwar emotions.

Despite this shift in diplomatic strategy, however, U.S. officials remain determined to get Japan to strengthen its defensive armor to help offset the burden of American military commitments in the Pacific. Continued Japanese reluctance, well-placed sources in the administration and Congress suggest, could emerge as the area of most serious conflict in the broad political and economic relations between the two nations.

While Japanese Prime Minister Zenko Suzuki pledged greater efforts on the defense front during White House summit talks with President Reagan in May, Suzuki's Cabinet has placed its priority on fiscal austerity, aimed at cutting back on Tokyo's massive government deficits. The scarcity of public funds, combined with the absence of a clear popular mandate for expanded defense programs, indicate that Tokyo's military spending will continue to undershoot Washington's objectives.

The flash point in two-way ties could come, these sources suggested, when Congress begins deliberations next year on the 1983 budget and politicians are obliged to support the large-scale military spending program endorsed by the Reagan administration at the expense of pork-barrel spending programs favoring their constituents.

"No American politician likes to stand up and cut spending on issues of immediate importance to constituents for defense [spending]," said a senior U.S. government official. Congressmen, he explained, "will look carefully at allies who benefit from the U.S. strategic umbrella and what they are willing to do for themselves."

While U.S. allies in NATO are also likely to come under fire for their reluctance to increase military expenditures, Tokyo presents a larger target because of what is viewed here as Japan's overwhelming advantage in economic relations with the United States. Tokyo's reticence, this official suggests, could again ignite complaints from congressional critics and elements in the America public who charge that Japan is taking a "free ride" on the back of heavy U.S. military commitments.

In attempting to put the best possible face on relations with Tokyo while keeping up strong behind-the-scenes pressure, Reagan administration officials say they hope to persuade the Japanese to amass an effective fighting force -- capable of defending the Japanese home islands, the sea lanes and airspace around Japan from conventional attack -- by 1990.

Relations reached an acrimonious impasse late last year when former defense secretary Harold Brown and other Carter administration officials were led to believe that Tokyo would increase its defense spending in 1981 by at least 9.7 percent. The final figure was 7.6 percent, and Brown charged that Japan was shirking its responsibilities.

In contrast, the Japanese Cabinet's announcement in June of plans to hold the figure for 1982 to 7.5 percent barely raised a ripple of protest in Washington.

"We've found it more useful to discuss matters quietly and out of the limelight," said another seniro U.S. government official. "It's pretty obvious that the previous administration not only did not get anywhere" in talks with the Japanese, but, he said, they "went a couple of steps back."

This shift to the practice of "quiet diplomacy" with the Japanese, the official suggested, seeks to create the impression that Japan will not be singled out from among Washington's other Western allies in NATO for special criticism.

U.S. officials said talks between Secretary of State Alexander M. Haig Jr. and Japanese Foreign Minister Sunao Sonoda in New York last month marked a "turning point" in the common assessment of the dangers posed by Moscow's military presence in the Pacific.

But while Haig and Sonoda, "saw eye to eye on the strategic situation," the official said, "changes won't take place overnight." The United States, he explained, recognizes the problems the Japanese face in building a national consensus on defense after more than three decades of postwar pacificism. The Soviet buildup around Japan has forced the Japanese to shed some of their previously intractable antiwar sentiments, but "too hasty a change in policy has the danger of leaving public opinion behind," this official said.

The Reagan administration's softer line on defense, U.S. officials suggested, is intended to give politicians in Tokyo, in the unobtrusive style of Japanese politics, more flexibility in creating the necessary national consensus on defense free from the kind of public opinion backlash overt American pressure can arouse in Japan's consensus-seeking society.

"The point [on defense] has been taken by the Japanese," the senior U.S. official said, "and we donht have to remind them time and time again." The fact that the U.S. intention to talk more softly on military issues with Tokyo is shared at the highest levels of the Reagan aministration was indicated, according to this official, when the president chose not to bring up the subject in private discussions with Suzuki at the Ottawa summit late last month.

Officials here are privately concerned, however, that Tokyo may be wrongly interpreting the signals from Washington as an excuse for complacency rather than a message that Japan must begin in earnest to undertake a more substantial burden in ensuring its own defense.

"Often the Japanese only think about what will make the Americans happy this year and not about the long-term importance of their defense policies," one administration official said.

"The problem we face," explained a senior U.S. official, "is that we are meeting longer range strategic requirements in view of the cumulative effects of higher Soviet spending [on defense] . . . and the political requirements in seeing some equity in sharing the burden to meet the challenge."