Just weeks ago, American diplomats thought Sudan was on the verge of a breakthrough agreement that would advance its transition from military dictatorship to full-fledged democracy, delivering on the soaring promise of the country’s revolution in 2019.

Sudan had become an important test case in President Biden’s core foreign policy goal of bolstering democracies worldwide, which in his view weakens corrupt leaders and allows nations to more capably stand as bulwarks against the influences of China, Russia and other autocratic powers.

But on April 23, the same American diplomats who had been involved in the negotiations in Sudan suddenly found themselves shutting down the embassy and fleeing Khartoum on secret nighttime helicopter flights as the country spiraled into a potential civil war.

Biden administration officials and their partners are now struggling to get two warring generals to stick to tenuous cease-fires and to end hostilities, as foreign governments evacuate civilians amid fighting that has left at least 528 dead and more than 330,000 displaced. The actual toll is almost certainly much higher than those Sudanese government numbers.

An urgent question at the heart of the crisis is whether the United States miscalculated how difficult it would be to introduce democracy in a country with a long history of military rule, and the risks of negotiating with strongmen who talk about democracy but never deliver.

Critics say the Biden administration, rather than empowering civilian leaders, prioritized working with the two rival generals, Gen. Abdel Fattah al-Burhan, the head of Sudan’s army, and Lt. Gen Mohamed Hamdan, a paramilitary chief, even after they carried out a military coup together in 2021.

Senior American diplomats “made the mistake of coddling the generals, accepting their irrational demands and treating them as natural political actors,” said Amgad Fareid Eltayeb, an adviser to Sudan’s deposed prime minister, Abdalla Hamdok. “This fed their lust for power and their illusion of legitimacy.”

And some analysts ask whether U.S. officials have a cleareyed approach to carrying out Mr. Biden’s global push for democratic resilience.

The violence in Sudan is creating exactly the kind of power vacuum that Mr. Biden’s aides had hoped to avoid. Russian mercenaries of the Wagner Group are among the players already trying to fill the gap, current and former U.S. officials say.

“If this fighting continues, there’s going to be a great temptation among outside actors to say, ‘If these guys are going to fight it to the death, we better get in there, because we would rather have this guy, or this institution, win,’” said Jeffrey D. Feltman, a former U.S. envoy to the Horn of Africa who worked on negotiations for civilian rule.

“If you don’t get to a cease-fire, not only do you have the misery of these 46 million people,” he added, “you have a higher temptation for outsiders to start hypercharging the fighting by direct intervention.”

Mr. Hamdok has said civil war in Sudan would make the conflicts in Syria, Yemen and Libya look like “a small play.”

The State Department and the White House declined to comment.

The White House’s Africa strategy paper, released in August, asserts that “by reaffirming that democracy delivers tangible benefits,” the United States can help limit the influence of “negative” outside nations and nonstate groups, reduce the need for costly interventions and help Africans determine their own future.

For the United States, the effort to prevent Sudan’s potential return to despotism is an unlikely role after decades in which the country was largely known for mass atrocities and as a haven for terrorists, including, for nearly five years in the 1990s, Osama bin Laden. In 1998, President Bill Clinton even ordered a missile strike on a pharmaceutical plant in Khartoum that he said Al Qaeda used to make chemical weapons, although that intelligence was later questioned.

It was not until October 2020, a year after the revolution, that President Donald J. Trump officially repealed the country’s status as a state sponsor of terrorism after Sudan normalized its relations with Israel.

“Today, a great people of Sudan are in charge,” Mr. Trump said. “New democracy is taking root.”

Mr. Feltman and other former and current U.S. officials say supporting democracy should still be the cornerstone of American policy in Sudan, given the aspirations expressed in protests that led to the ouster in 2019 of President Omar Hassan al-Bashir, the dictator of 30 years. Congressional leaders are now calling for Mr. Biden and the United Nations to appoint special envoys to Sudan.

The setbacks in Sudan follow other democratic disappointments in northern Africa, including a military counterrevolution in neighboring Egypt a decade ago; nearly 10 years of political anarchy in Libya, another neighbor of Sudan, after its dictator, Col. Muammar el-Qaddafi, was overthrown; and a recent return to one-man authoritarian rule in Tunisia after a decade as the only country to emerge from the 2011 Arab Spring with a democratic government.

Mr. al-Bashir’s downfall four years ago led to joyous displays from Sudanese who hoped that democracy might take root their country despite its failures elsewhere in the region. After several months of junta rule, Sudan’s military and civilian leaders signed a power-sharing agreement that created a transitional government headed by Mr. Hamdok, an economist. The plan envisioned elections after three years.

However, a council formed to help manage the transition was “a bit of a fig leaf,” since it had more military than civilian members, Susan D. Page, a former U.S. ambassador to South Sudan and a professor at the University of Michigan, said in a post on her school’s website. Important civilian voices were excluded, a problem that would persist into negotiations this year.

After the military coup in October 2021, the United States froze $700 million in direct assistance to Sudan’s government and suspended debt relief, while the World Bank and the International Monetary Fund froze $6 billion in immediate assistance and plans to forgive $50 billion of debt. Other governments and institutions, including the African Development Bank, took similar steps.

Ned Price, the State Department spokesman at the time, said that “our entire relationship” with Sudan’s government might be re-evaluated unless the military restored the transitional government.

Even as coup rumors circulated that October, American officials had warned General Hamdan that he would face “specific consequences” if he seized power, a former senior U.S. official said. But after the coup, American diplomats under Molly Phee, the department’s top Africa policy official, decided to try to work with the generals rather than enter into confrontation with them.

The U.S. official declined to specify the proposed sanctions against General Hamdan but said they broadly targeted his personal wealth, much of it held in the United Arab Emirates — a war chest that experts say was critical to building up a military force that has been unleashed in the current fighting.

Pressure to punish the generals came from senior members of Congress. Senator Chris Coons, a Democrat of Delaware on the Senate Foreign Relations Committee’s subcommittee on African affairs, co-wrote in a Foreign Policy article in February 2022 that the Biden administration should impose a “comprehensive set of sanctions on the coup leaders and their networks” to weaken their grip.

Speaking to reporters during a trip to East Africa with Secretary of State Antony J. Blinken in November 2021, one senior State Department official said the generals had indicated that they were willing to again share power with civilians. The official, who insisted on anonymity to talk about the negotiations, said that withholding aid might not do enough to pressure the generals, and so the administration had appealed to their sense of an honorable personal legacy, among other things.

Cameron Hudson, who served as the chief of staff to successive U.S. presidential special envoys for Sudan, called that approach a mistake.

“They put too much faith in what these generals have been telling them. These guys have been telling us what we want to hear since they agreed to civilian rule” after Mr. al-Bashir’s ouster, Mr. Hudson said. “There was supreme confidence in the State Department that we were on the cusp of a breakthrough agreement.”

Washington’s willingness to bargain with the generals after the coup had the effect of legitimizing them, Mr. Hudson said.

The United States also failed Mr. Hamdok before the coup, he added, when bureaucratic inertia slowed the disbursement of economic aid meant in part to show the benefits of civilian rule.

That left Mr. Hamdok all too vulnerable.

The coup left Mr. Feltman feeling betrayed. The generals had personally assured him hours before they arrested Mr. Hamdok that they would not seize power, he said.

But even if the United States had imposed sanctions on them, “I’m not sure it would have made much difference,” he said. “The two generals see this as an existential battle. If you are in an existential battle, maybe you are annoyed by sanctions, but it won’t stop them going after each other.”

The first breakthrough after the coup came in December 2022, when the United Nations, the African Union and a regional bloc brokered a deal to transition Sudan to civilian rule in a matter of months.

But enormous issues still had to be resolved, notably how quickly General Hamdan’s Rapid Support Forces would be merged with the regular military, and who would report to a civilian head of state. The work of bridging those differences fell largely to the dominant foreign powers in Sudan: the United States, Britain, Saudi Arabia and the United Arab Emirates.

Even though Saudi Arabia and the Emirates are authoritarian monarchies, they profess to want democracy in Sudan.

But as negotiations advanced, the gap between the two generals grew. Military reinforcements from both camps began to enter Khartoum.

In late March, American and British diplomats presented the generals with proposals intended to bridge their biggest differences. Instead, the plan seemed to sharpen tensions. Weeks later, on April 12, General Hamdan’s forces seized control of an air base 200 miles north of Khartoum, in the first public sign that the years of diplomacy were culminating in war.

Three days later, the fighting began.