If life is full of surprises, life in the Middle East is full of shocks. Even by that standard, the fall of Damascus is a stunner. Until two weeks ago, nothing in the region seemed more permanent than Syrian President Bashir al-Assad’s hated regime.

And yet, on Sunday, anti-government militia forces culminated a lightning offensive across the country, seizing Syria’s capital as Mr. Assad’s army melted away. Mr. Assad has fled the country, thus ending his family’s brutal half-century dynasty.

To Mr. Assad we say: Good riddance. The speed of his downfall is testament to the illegitimacy of his awful rule, marked by mass executions, torture and support for terrorism. During the past 13-plus years of civil war, the regime depended on Russia, which carried out devastating airstrikes, Iran and its Lebanese proxy group, Hezbollah. But Russia had withdrawn troops for its war with Ukraine, Hezbollah has been decimated by its war with Israel, and Iran, also weakened by clashes with Israel, wrote off Mr. Assad.

For Syrians, the nightmare of Mr. Assad’s misrule is finally over. But euphoria over his ouster must be tempered by questions over what comes next.

It might be tempting to assume that anything is better than Mr. Assad. That would be a mistake. Hayat Tahrir al-Sham, or HTS, the main rebel group behind the current offensive, is an al-Qaeda offshoot that once had links to the Islamic State. Deemed a terrorist organization by the United States, HTS is led by Ahmed al-Sharaa, also known as Abu Mohammed al-Jolani, who fought U.S. troops in Iraq. Mr. Jolani has lately been trying to project a more moderate image and convince Syrians that all ethnic groups would be welcome in the post-Assad Syria. In the Idlib region of northwestern Syria, which it controls, HTS has provided protections for women and religious minorities but has also been accused of human rights abuses and authoritarian rule.

How Mr. Jolani handles the immediate transition after Mr. Assad’s ouster will give clues as to which face of HTS to believe. Mr. Jolani needs to publicly commit to upholding Security Council resolution 2254, adopted in 2015, which calls for a ceasefire, a transitional authority involving all Syrian warring factions except terrorist groups, and U.N.-supervised elections after 18 months.

Syria’s economy is a wreck, and it needs international assistance to rebuild.

That help should be made contingent on Mr. Jolani’s commitment to an inclusive, pluralistic Syria.

There should also be no wholesale retribution against old regime officials or the military.

If Mr. Assad and his most-complicit aides are ever again in the country, the new government would be perfectly reasonable to hold them accountable for the crimes of his 24-year rule.

But his soldiers were mostly conscripts. And some civil servants from the old regime will be needed to help necessary state institutions continue to function. Mr. Assad’s erstwhile prime minister has already offered an olive branch, saying, “We are ready to cooperate with any leadership the people choose.” Mr. Jolani and the HTS should take up the offer.

Syria might seem far removed from U.S. interests. Before Mr. Assad’s fall, President-elect Donald Trump posted; “DO NOT GET INVOLVED!” But America is involved. Some 900 U.S. troops and an undisclosed number of military contractors are operating in northeastern Syria near Iraq, battling the Islamic State and backing Kurdish forces fighting the Assad regime. Iranian-backed militants using rockets, missiles and drones have attacked U.S. troops 125 times since October 2023.

Mr. Assad possessed prohibited chemical weapons, which he used against both rebels and — mostly — civilians. Securing that stockpile and keeping it out of the wrong hands is a paramount U.S. concern.

Syrian unrest has created the world’s largest refugee crisis, with nearly 6 million displaced Syrians living in neighboring countries and Europe. The European Union hosts more than 1 million Syrian refugees, mostly in Germany, and a backlash against migration has helped fuel the rise of far right parties, one of which is a contender for power in German elections planned for February. France is in the throes of a political crisis with a national assembly paralyzed by the far right National Rally party.

Even before Mr. Assad’s ouster, some European politicians were angling — prematurely — to declare the Syrian conflict over so they could force the refugees home.

They must not act too quickly now until Syria’s situation clarifies.

The Middle East badly needs a success story: a pluralistic, democratic Arab country committed to upholding human rights.

For more than 50 years, Syria under the Assad family regime epitomized so much that is wrong about the region. With engaged diplomacy, the United States can help write a brighter next chapter for this strategically located, and long-suffering, country.