Benazir Bhutto’s assassination leaves Western policy toward Pakistan facing its greatest test since the September 11, 2001, attacks. At a stroke, the lynchpin of American and British plans to bring stability to the country has been removed. President Musharraf allowed Bhutto to return to her homeland and contest the planned parliamentary election only after coming under heavy pressure from his key allies.

Washington and London wanted Bhutto back, and they wanted her to become prime minister once again. Mr. Musharraf had grown so unpopular that his government risked becoming wholly ineffectual. The answer was to unite Pakistan’s secular, pro-Western politicians in one administration. This would serve the dual purpose of isolating the Islamist radicals and creating a new government capable of fighting them effectively.

The West hoped that Bhutto would win the parliamentary election at the head of her Pakistan People’s Party and then serve as prime minister in coalition with Mr. Musharraf, now a civilian leader. Meanwhile, the president’s successor as army chief, General Ashfaq Pervez Kiyani, would fill the third key position in the Pakistani state.

How this triumvirate would have worked is anyone’s guess. Throughout its 60-year history, Pakistani politics has been crippled by the failure to find a balance between the posts of president, prime minister, and army chief.

Bhutto was the only pro-Western politician who commanded genuine mass support. Hence she was a uniquely valuable ally for America and Britain. All these plans have now gone by the board. Pakistani politics is more unpredictable and dangerous than at any time since September 11.

Mr. Musharraf may be forced to reimpose the state of emergency that he only lifted a few weeks ago. If so, the parliamentary elections set for January 8 will probably be postponed, perhaps indefinitely. Bhutto’s supporters have already accused the regime of organizing her assassination, a claim that is widely believed and could stir further unrest.

America and Britain are now faced with two options, both unpalatable. The first would be to back Mr. Musharraf, their old ally, even if he uses emergency powers to hang on and cancels the elections. This would leave them beholden to a deeply unpopular leader who is demonstrably incapable of stemming the rising tide of Islamist militancy.

The second option is to retain the goal of forming a broad administration, uniting all of Pakistan’s democratic forces. Nawaz Sharif, the former prime minister who Mr. Musharraf overthrew in 1999, could then be substituted for Bhutto’s role.

Washington and London will probably settle on this option.

BHUTTO’S POLITICAL LIFE

She served two times and was seeking a third term as premier.

• April 10, 1986: Returns from exile in London to lead the Pakistan People’s Party her father founded.

• December 1, 1988: Becomes first female prime minister of a Muslim nation after winning parliamentary elections.

• August 6, 1990: President Ghulam Ishaq Khan dismisses Bhutto’s government, citing corruption and failure to curb ethnic violence.

• October 19, 1993: Takes oath for second term as prime minister.

• November 5, 1996: President Farooq Leghari dismisses Bhutto’s second administration amid accusations of nepotism and undermining the justice system.

• April 14, 1999: Found guilty of corruption while out of country; conviction was later quashed, but she remains in exile.

• October 5, 2007: President Musharraf signs a corruption amnesty covering other cases against Bhutto, opening way for her return and possible power-sharing agreement.

• October 18, 2007: Returns to Pakistan, ending eight years of exile.

• December 27, 2007: Killed at a political rally in Rawalpindi.

– Associated Press