President Reagan returns to the United States from a week in the Sea of Japan riding a crest of enthusiasm at home. Perhaps it could be called ''the Grenada high'': a feeling among many Americans that their nation's restless might had at last been directed in some symbolic, useful manner. His political position appears akin to British Prime Minister Margaret Thatcher's after her spirited defense of the Falkland Islands, which she quickly converted into an electoral victory.
In Japan and South Korea, strategically more significant than Grenada, Mr. Reagan built on this Grenadian theme of the assertiveness of US might. The imagery of his visit to the Korean demilitarized zone - ''eyeball to eyeball with the communists'' - will likely be remembered as characterizing his journey.
Travel abroad helps a president's image. That was why Mr. Reagan's political strategists originally planned to have him make his reelection announcement at this point, on his return from the Far East. Mr. Reagan's popularity had already been on the incline much of the year, with spikes upward from events like his speech to the nation after the Grenadian invasion. By early this month he was showing gains in handling the economy, in foreign affairs, and overall in his job as president. From the paroxism of national anguish over the loss of US marines in Lebanon a few weeks ago, precipitating Mr. Reagan's biggest leadership crisis, the apparent turnaround for the President must be noted.
But the base for his rise in popularity was evident during last year's elections, when the Republicans did surprisingly well, considering the economy's troubles. The economic upturn this year has helped Reagan more than anything else.
Still, there are usually two sides to any political equation. The President's pluses are not without their minuses. The broad design of his foreign policy is generating its critics. The governing board of the National Council of Churches last week found troubling the ''tendency of the US to militarize international problems regardless of their social or political origin''; the ''confrontational posture between the US and the Soviet Union'' that ''fuels an arms race that threatens to engulf the whole world''; the US ''failure . . . to make a credible commitment to arms control and disarmament''; and the recent US tendency to ''subordinate human rights considerations . . . to strategic and economic interests.''
Americans are showing an ambivalence about Mr. Reagan's leadership. By 55 percent to 37 percent they approved his handling of foreign affairs in the latest ABC News/Washington Post survey. But by an even wider margin they feared that the President is increasing the chances of war. Women, particularly, said they were more apprehensive about war.
In any event, Mr. Reagan returns home now even or ahead of his Democratic rivals Walter Mondale and John Glenn in trial 1984 match-ups for the White House. He appears about ready to settle his differences with Congress on fiscal 1984 spending this week, before Congress leaves Friday for the Thanksgiving recess. His new special envoy to the Middle East, Donald Rumsfeld, is on his first mission, attempting to contain Lebanon's disputes. With US firepower massed off the Lebanese shore, that region remains a powder keg - and, contrasted with Grenada, the US public remains perplexed over the purpose of US involvement there.
Mr. Reagan in Asia urged Japan to spend more on defense and open its markets wider to US goods and services. In South Korea, he was embarrassed to discover President Chun Doo Hwan had put potential troublemakers under house arrest. Mr. Reagan's close embrace of the South Korean leader, intended to signal US-South Korean military solidarity, troubled those inside and outside Korea concerned about the sluggish pace of restoration of human rights under President Chun.
Mr. Reagan returns to the White House politically more comfortable. The lines of a 1984 presidential race look clearer: The big issues of bread and butter, peace and war, will be even bigger this time around. Mr. Reagan's greater assurance seems to be pushing the Democrats to rally more around a single candidate, at the moment to Mr. Mondale's advantage. The campaign only seems to lack Mr. Reagan's announced candidacy.