The world will little note nor long remember the 12th Economic Summit, for all of the U.S.-orchestrated hoopla about democratic unity in the face of state-supported terrorism. President Reagan had barely returned to the White House before the Japanese government asserted that adherence to a summit declaration calling for six specific measures to identify and deter international terrorism is purely voluntary. The French also have declined to commit themselves to specific actions and stand firmly with the Italians and West Germans against economic sanctions. So much for democratic unity.

Nonetheless, there is much to be said for Reagan's overall performance in Tokyo, where he appeared effective and self-assured in his dealings with other national leaders and the news media. Those of us who have tracked Reagan on his trajectory from second lead in "Bedtime for Bonzo" to master politician of the western world were struck by the difference between Reagan's showing in Tokyo and the impression he made at earlier summits.

Six years ago, in Ottawa for his first summit, Reagan was so overwhelmed by his appearance on the world stage that he told old welfare jokes to bewildered leaders of the assembled democracies, who must have wondered how anyone with such a thin grasp of world affairs could be elected president. On Air Force One returning from Ottawa, the exhausted and defensive president was so incoherent that two reporters who interviewed him turned to aides for an interpretation.

The next summit, at Versailles in 1982, wasn't much of an improvement. Reagan's plea to U.S. allies not to sell gas-pipeline equipment to the Soviets was ineptly presented and promptly ignored. The trip became known to reporters as "Naptime at the Vatican" because the president, again inflicted with a punishing schedule, fell asleep on television while meeting with the pope.

In contrast, a well-rested Reagan, buoyed by economic prosperity and secure in his domestic popularity, gave a commanding performance at Tokyo, where he was the center of attention. The lasting value of summit declarations may be questionable, but it is incontestable that summit leaders largely debated the U.S. agenda on world affairs and economics, creating what White House Communications Director Patrick J. Buchanan called "a Reagan summit."

After Ottawa and Versailles, White House aides were saddled with the embarrassing task of trying to explain why Reagan was the only leader who didn't hold a post-summit news conference. The real answer, which they didn't want to admit, was that Reagan would have been hopelessly at sea on most of the issues. But in Tokyo, Reagan seemed almost as much at home at a news conference as he once did on a movie set, crisply answering 24 questions in 23 minutes. I don't share Reagan's belief that Libyan terrorism can be overcome by air power, but the president made a forceful case for his policy. For once, it almost seemed as if he enjoyed being questioned.

The president's improvement on the international stage reflects most of all the accumulation of experience. Reagan resists theoretical information but learns by doing. He pays attention to what people say to him and understands their political needs. After six summits, he knows the leaders very well and the issues well enough.

It seems to me that Reagan has also become more realistic, at least on the margins, about what can be accomplished in international affairs. He stroked the French, rather than confronting them. He put a diplomatic gloss on the toothless terrorism accord, recognizing that it was the best that he could get. And while Reagan delivered his usual paean of praise to free markets, his Treasury secretary, James A. Baker III, was pushing through the summit's only tangible economic accomplishment: a plan for government intervention to stabilize world currencies in crisis situations.

Reagan deserves credit for such realism and the smoothness of his effort at Tokyo. He won grudging praise from the most difficult of constituencies, the French and the news media. When Reagan goes abroad, his aides don't have to apologize for him anymore.

Reaganism of the Week: Describing the meeting between French President Francois Mitterrand and Reagan in Tokyo last Tuesday, White House spokesman Larry Speakes quoted the president as saying: "Let this be the first day of the rest of our lives."