Afghanistan’s beleaguered government will be on its own against the Taliban once American forces withdraw in September. The strength of the insurgency, combined with internal disarray in the administration of President Ashraf Ghani, suggests Kabul could find it hard to hold out for long.

U.S. Secretary of State Antony Blinken flew to Kabul to reassure Afghan leaders on Thursday that America’s “partnership is enduring,” despite President Biden’s determination to extricate the U.S. from a 20-year conflict.

But many in the Afghan capital said they fear the pullout sets the stage for a renewed civil war, possibly one in which Afghanistan’s neighbors India, Russia, China and Iran will be playing a much more significant role.

“We don’t know if Afghanistan will become the new Syria,” said Timor Shah Mohseni, a 48-year-old shopkeeper in central Kabul.

Even before this week’s decision by Washington, tensions had been mounting within the uneasy coalition of Afghan power brokers—led by Mr. Ghani—brought together by their mutual hostility to the Taliban and now running the national government.

“Kabul has been fractured. The different factions that Ghani needs to keep together have begun to read the tea leaves,” said Vali Nasr, a professor at the Johns Hopkins School of Advanced International Studies who advised the Obama administration on Afghanistan. “Everyone knows that the government in Kabul in its current form is untenable without U.S. military support.”

The Taliban are believed to control or contest more than half of Afghanistan’s districts. The Afghan government, which has stopped releasing military casualty statistics to avoid demoralizing the troops, said this week that its security forces are conducting close to 98% of operations independently and are fully capable of defending the country on their own.

Still, Fahim Ahmadi, a mobile-phone repair technician in Kabul, said he feared the worst. “The roots of this government are weak,” he said. “I am worried about the start of another civil war in the country.”

For Afghan women, a flagship cause for a generation of Western aid workers, the future looks particularly dire, as many hard-won freedoms look likely to be rolled back.

Fawzia Koofi, one of four women on the Afghan government team in peace negotiations with the Taliban, has been calling for months for women to have a more significant stake in the peace process.

“We are already the victims, not only losing lives but losing opportunities,” said Ms. Koofi, referring to a recent spate of unclaimed assassinations targeting female activists and journalists. “It not only puts the achievements we have made so far in danger, but international security as well.”

In two major Cold War conflicts—America’s involvement in Vietnam and the Soviet occupation of Afghanistan—considerable time passed between the final withdrawal of foreign troops and the fall, respectively, of Saigon and Kabul. (Henry Kissinger, who negotiated with North Vietnam in Paris, described that lag as a “decent interval” that was meant to allow Washington to save face.)

In both cases, cutoffs in financial and logistical assistance—due to Congressional action in the case of South Vietnam and the unraveling of the Soviet Union for Afghanistan—also contributed to the collapse of the local governments and security forces.

However, a loss of U.S. air support for Afghan government troops would likely allow the Taliban to seize and keep major cities in the short term. The insurgent group overran a handful of provincial capitals in recent years, only to be driven out by U.S. and Afghan special operations forces.

“All the air force, airstrikes, rockets, maintenance and support is done by the U.S. So, when they are not here, who is going to do that?” said Masoud Andarabi, who served until last month as interior minister. “That’s going to be a huge challenge.”

The regime of Soviet-installed President Mohammed Najibullah lasted some three years after the last Soviet soldier left the country in 1989, and imploded only after a series of defections by his senior military commanders, most notably Uzbek warlord Abdul Rashid Dostum, who served as vice president in Mr. Ghani’s administration until last year.

Mr. Ghani’s current government looks even shakier than Mr. Najibullah’s did, some diplomats and analysts say.