WASHINGTON (AP) - A landmark treaty signed 40 years ago Tuesday by 61 countries is turning out to be more successful than predicted in keeping the nuclear weapons club small.

But as a State Department official cautioned at the time in a newly published memo, the United States has limited leverage to slow down the spread of bombs.

"I don't think we are reining in technology," said Richard Rosecrance, a Harvard adjunct professor who was on the State Department's policy planning staff in 1968 when he wrote the memo for Secretary of State Dean Rusk: "That's very difficult to do. But we may be able to rein in intensions."

Rosecrance, who predicted in the memo that "nuclear proliferation will continue," said in an interview Tuesday that "when countries get close they usually hold back building a bomb."

Underscoring the problem, though, he wrote even 40 years ago that the basic secrets of bomb technology and manufacture were already widely known. And, he said, a nation could achieve an advanced state of nuclear "pregnancy" while remaining within the strictures of the treaty.

In 1963, President John F. Kennedy predicted at a news conference that he saw "the possibility in the 1970s of the president of the United States having to face a world in which 15 or 20 or 25 nations may have nuclear weapons."

"I regard that as the greatest possible danger," Kennedy said. He was not alone in that forecast.

Today, the nuclear club has nine members: The United States, Russia, Britain, France, China, India, Pakistan, North Korea and, it is generally believed, Israel.

The treaty commits the now 188 nations that have signed the accord not to acquire nuclear weapons. The five countries that had already held nuclear tests, the United States, Russia, Britain, France and China, were to pursue "effective measures" toward nuclear disarmament.

Meanwhile, India, Pakistan and Israel refused to sign the treaty.

There is virtually no movement on the disarmament front, while concern about a breakout is focused now on Iran, which claims its nuclear program is not aimed at building a bomb, and North Korea, which in 2003 became the only country to defect from the treaty. The Stalinist regime recently took some big steps, though, toward revealing essential elements of its program.

The two countries are not the only ones with the technology that very quickly can be translated to nuclear weapons. Japan and Germany are known to have the elements of a nuclear weapons program. Canada could have been an early member of the club but chose not to go ahead. South Africa had six bombs but disassembled them. Libya abandoned its program.

President Bush, marking the 40th anniversary of the signings in Washington, London and Moscow, issued a statement calling for strong action to strengthen measures designed to stem the proliferation of nuclear weapons technology.

He called the treaty, which took effect March 5, 1970, "a key legal barrier to nuclear weapons proliferation."

In an interview, Daryl Kimball, executive director of the private Arms Control Association, called the treaty "imperfect."

"But it has been extremely successful and continues to be invaluable," he said.

The challenge, he said, is to strengthen the treaty's implementation and to plug loopholes in it.

Back in June 1968, with the headline "After NPT, What?," Rosecrance, who was a member of the State Department's policy planning board, prepared a memorandum for Rusk that said countries are entitled under the treaty "to proceed a considerable distance" toward a potential military nuclear program.

This is true, he said, partly because prestige is a major motivation for nuclear status.

Documents, published by the National Security Archive of George Washington University, detail among other hurdles, India's resistance to the treaty with "China at her back and Pakistan lurking on the sidelines" and Italy's unhappiness at the "second-class status" of non-nuclear states.

Still, there was considerable optimism in 1968 that a historic point had been reached. "The signing of this treaty keeps alive and keeps active the impulse toward a safer world," President Lyndon B. Johnson said as Rusk signed the accord for the United States.