ince the end of the cold war in the early 1990's, there have been two Americas, existing side by side: the America that is the world's only superpower, dwarfing every other nation in its economic and military might, and the America that learned in one deathly hour on Tuesday that no amount of power can provide protection against an enemy with limited means but a ruthless determination.

For years that vulnerability has been a concern of defense experts in the Pentagon and elsewhere who have studied what some have called "asymmetric warfare" — a 21st- century phrase for a concept that has been around in warfare since David and Goliath and has found its most menacing expression in the modern world in the suicide bomber.

But it has also been studied, and celebrated, by many of the terrorist groups that will now be on the list of suspects in the attacks on the World Trade Center and the Pentagon.

One of those certain to be high on that list, Osama bin Laden, has made America's helplessness in the face of terrorism a rallying cry, one he has used repeatedly to taunt the United States and to draw new recruits to his ranks of suicide bombers. People close to Mr. bin Laden in Afghanistan, where he lives, today denied responsibility for the attacks.

The theme of American helplessness, applied as often to Israel as to the United States, is common among the leaders of Islamic militant groups like Mr. bin Laden's Al Qaeda, as well as two groups that operate in the Palestinian territories, Hamas and Islamic Jihad, and the Iran-backed Hezbollah, which operates from Lebanon.

Although investigators may well absolve these groups of the New York and Washington attacks, there is little doubt that the officials will be examining the threats that the leaders have issued against the United States. None of the threats is likely to draw more attention than what Mr. bin Laden had to say about America's susceptibility to terrorism in a two-hour videotape delivered to a Kuwaiti newspaper this summer.

On the tape, which proliferated rapidly on Islamic Web sites and in mosques and bazaars across the Muslim world, Mr. bin Laden seemed to gloat as he spoke in Arabic of future attacks on American targets that he said would dwarf those he has directed in the past. "With small capabilities, and with our faith, we can defeat the greatest military power of modern times," he said at one point. "America is much weaker than it appears."

At one point, the Saudi Arabian- born Mr. bin Laden seemed to hint at a suicide attack in the United States. Over pictures of Sheik Omar Abdel Rahman, the Egyptian-born Muslim cleric who is serving a life sentence in the United States for his role in the 1993 World Trade Center bombing, Mr. bin Laden described the cleric as "a hostage in an American jail." He added, "We hear he is sick, and that the Americans are treating him badly." Then, addressing his followers, he shifted straight into an exhortation to martyrdom. "You will not die needlessly," he said. "Your lives are in the hands of God."

Since the tape surfaced in June, there have been other hints that a new bin Laden attack might be imminent. Abdel Bari Atwan, editor in chief of Al Quds al Arabi, an Arabic- language newspaper published in London that has kept a close watch on Mr. bin Laden and his pronouncements, said today in an interview with the BBC that Islamic militants in touch with the newspaper had hinted that a major attack was under preparation, without giving any hint of when or where.

Elsewhere on the tape, Mr. bin Laden called on his followers to prepare new suicide attacks to avenge Palestinians who have been killed in the past year's violence with Israel, Without specifying whether those attacks would be mounted against Israeli targets or elsewhere.

At first, when the bin Laden tape surfaced it attracted only passing attention, partly because large passages were spliced from earlier bin Laden interviews and tapes.

But intelligence officials in Washington see it as the fullest exposition yet of the bin Laden strategy, and a rough road map to where Al Qaeda, his group, might be headed.

One of the characteristics that has set Mr. bin Laden aside among terrorist leaders has been the way he has coupled the furtiveness inherent in his deadly trade with warnings that new attacks may be in hand.

On several occasions, he has done this by releasing tapes or interviews alluding to new attacks shortly before they have been carried out.

He did this with a tape that surfaced in the Middle East shortly before two Arab-speaking suicide bombers in a fiberglass skiff attacked the American destroyer Cole last October in Aden harbor in Yemen, killing 17 American sailors. In the latest tape, he promised that the Cole attack was only a preliminary to new strikes. Although recent weeks have seen the State Department issue a flurry of alerts, and temporarily close several embassies considered possible targets, experts were not convinced that the tape presaged a major strike, partly because much of it had been taken from tapes made as much as three years ago.

On the tape, Mr. bin Laden read a chilling poem with themes that have a powerful resonance among Muslims with the grievances against America.

Principal among these grievances is Israel, and the support it draws from the United States — a shift in emphasis, according to intelligence experts. In the past, Mr. bin Laden, one of more than 50 children of a Yemeni-born migrant who made a vast fortune building roads and palaces in Saudi Arabia, has made his principal obsession driving American troops from the Arabian peninsula, site of Mecca and Medina, Islam's holiest sites.

But this time, "purifying" the holy places seemed to take second place behind supporting Palestinians. Over scenes of some of the harshest incidents, like footage of a young Palestinian boy shot in his father's arms as they sought shelter from crossfire in Gaza, Mr. bin Laden urges the killing of Americans and Jews: "We will see again Saladin carrying his sword, with the blood of unbelievers dripping from it."