WASHINGTON - As US senators debated the fate of a nuclear test ban treaty yesterday, the government of a far-off nuclear power, Pakistan, was overthrown in a military coup.

That the two events collided at the same hour sounded too fantastical at first, the oddness of senators debating about nuclear tests as the whiff of danger emerged in Pakistan, where only last year the government followed India's nuclear tests with a test of its own that defied the world.

But as the day wore on, and as clever senators wove into their test-ban speeches lines about the potential instability in Pakistan, the uncertainties of the post-Cold-War world were laid bare. Yesterday afternoon, Washington and Islamabad could have been an opening chapter of a Tom Clancy novel.

The two events were linked by subject, by timing, by the fears raised in the nearly empty Senate chamber and the shots heard in some Islamabad neighborhoods.

Even before the coup, nuclear newcomers such as India and Pakistan, asked by Washington to adopt the Comprehensive Test-Ban Treaty, saw US senators reluctant to take the same step. And despite the Pakistani situation and the ominous move by neighboring India to put its soldiers on high alert, senators gave no sign they would support ratification of the treaty.

But the events in South Asia turned the Senate debate into more than a theoretical exercise.

Now there was an unfolding internal crisis of a country with a nuclear bomb that borders another country with a nuclear bomb, with both countries sharing a history of enmity. They had fought three wars prior to their back-to-back nuclear tests last year, and Clinton intervened this year to get Pakistan's prime minister to withdraw troops from the disputed Kashmir - an act that enraged Pakistani military leaders.

The most serious question was what would happen next in Pakistan and India, and what the US government should do to try to stabilize the situation. In South Asia, some worried that the lack of US ratification for ending nuclear tests added another element of danger to an unpredictable situation.

''Now everything is uncertain in Pakistan,'' said Sugata Bose, a Calcutta native and professor of diplomacy at the Fletcher School of Law and Diplomacy at Tufts University. ''I have been worried ever since the nuclear tests of last year, and today's development adds to the worries.''

Bose, who returned to Boston on Friday after spending nearly two weeks in India to watch the electoral campaign, said the ''Republican opposition to the test ban treaty hasn't helped in India or Pakistan. It's been a contributory factor to this kind of uncertainty and instability. If the US had ratified it, it would have focused the minds of India and Pakistan.''

In his discussions with some members of India's new government, Bose said he ''heard from many people, `Why should we waste our time debating this issue when the US doesn't seem to be prepared to ratify?'''

In Islamabad, General Pervez Musharraf said the military seized power after Prime Minister Nawaz Sharif's government ''systematically destroyed'' state institutions and drove the economy toward collapse. The ouster came hours after Sharif tried to fire the general, but Sharif's unpopularity had been rising in the military following orders to withdraw militants from Kashmir and to aid the hunt for terrorist Osama bin Laden.

With Sharif ousted, President Clinton loses an ally in his efforts to help stabilize relations between India and Pakistan. In addition, Musharraf is viewed with great suspicion in India and believed by some government leaders as an instigator of the Kashmir conflict this year.

Meanwhile, halfway around the world, some US senators cited the Pakistani coup as proof that the test ban treaty should be ratified.

''It makes sense for us to consider what's happening around the world,'' said Senator Joseph Biden, a Delaware Democrat and ranking minority member of the Foreign Relations Committee.

He said Senate action on the treaty ''does have impact upon'' the Pakistani situation. Without Senate ratification, ''we lose any leverage we would have to impose upon the Pakistanis'' to stop nuclear tests, Biden said.

But many Republicans, both moderate and conservative, said the flaws of the treaty were too great to ignore.

The treaty, which has been ratified by 26 of the 44 nuclear powers and needs ratification of all 44 before going into force, calls for the creation of an organization that will conduct on-site inspections of nuclear weapons. The organization also will keep watch for any nuclear tests through ground vibrations, air sampling, and infrasound tests.

But what happens to the US stockpile of weapons? That has been the main concern of Republican senators, who said the treaty would put the United States at risk by not allowing tests.

The United States voluntarily has not tested its nuclear missiles since the early 1990s, and instead embarked on a $4.5 billion program to monitor the stockpiles. Scientists debate the degree of erosion from the aging of the missiles, but no one disagrees that some components of nuclear weapons degrade over time. Of particular concern are non-nuclear parts on missiles, such as fuses.

''Am I certain that if we fired missiles that all the missiles would go off? No,'' said Theodore A. Postol, professor of national security policy at Massachusetts Institute of Technology. ''There's always uncertainty. ... But the question is, would you rather live in a world of uncertainty where you have a test ban treaty, or live in a world where you don't have a treaty. My view is the world with the test ban is a better world to live in.''

Postol said the Pakistani situation underscores the need for stricter measures guarding against the use of nuclear power - not just in Pakistan, but in every nuclear power.

''We ought to be very concerned about the long-range situation in Russia,'' he said. ''And I think the problems of India and Pakistan are particularly significant right now because you have two nuclear weapons states bordering each other with deep unresolved conflicts and a history of war.''