The resignation of a leader who toppled a democratic government nine years ago in a military coup might seem a cause for celebration — and it probably is. Even so, the departure of Pakistan President Pervez Musharraf, a longtime ally of the United States, is another ominous development during a period when the bellicose behavior of Iran and Russia is already making the world a very scary place.

That is because Pakistan is the only nuclear-armed Muslim nation, and a strong strain of Islamic extremism runs through its population. It has become a staging ground for terrorists trying to undermine the new democracy in Afghanistan and wreak havoc in India, as well as plot future attacks on America.

The United States and its allies have to be deeply concerned about political stability in the country, and the formation of a government that will restrain terrorism rather than nurture it.

General Musharraf’s resignation came as he confronted impeachment in a country that is in political and financial distress. In quitting, he voiced sadness “that Pakistan is sliding downhill very quickly.”

He incurred the wrath of many Pakistanis last year when he fired the chief justice, imposed emergency rule and suspended the constitution, leading to his party’s drubbing in elections in February. Since then, he has had little power to lead or serve usefully against terrorism.

Pakistan is crucial to the stability of that region, though, which is why U.S. taxpayers have pumped in more than $10 billion in aid, most of it military, since 2001. Such aid will no doubt continue, in an attempt to secure Pakistan’s cooperation. But it would probably serve America’s cause if more of it could filter down to help Pakistanis develop their economy and escape poverty.

Two leaders are now contending for power: Nawaz Sharif, the prime minister whom General Musharaff drove from office in 1999; and Asif Ali Zardari, widower of former Prime Minister Benazir Bhutto, who was assassinated last December. Looming in the background, the military will have a strong say.

It would be greatly in the interest of the United States if a stable and responsible ruling coalition emerged, one that would crack down on terrorism and, in particular, rein in Inter-Services Intelligence, the Pakistani spy agency linked to attacks on India.

The coming weeks will reveal whether we have another international crisis on our hands.