The Assad regime in Syria came toppling down in lightning speed after rebel forces led a days-long offensive across the country and captured the capital, Damascus, on Sunday, with President Bashar Assad ultimately fleeing to Russia.

How did it happen so quickly?

Some Syrian analysts, as well as the Biden administration, have pointed to Assad's chief backers -- Iran, Russia and Hezbollah -- as having been "weakened and distracted" in recent months. Israeli Prime Minister Benjamin Netanyahu in September said the killing of Hezbollah leader Hassan Nasrallah was a "historic turning point."

Other close watchers of Syria have also pointed to another key factor: a tiny white pill with a pair of interlocking half-moons on one side -- a synthetic and amphetamine that is wildly popular in the Middle East known as Captagon.

Experts say that the drug trade stemming from Syria, the world's largest Captagon supplier, helped hasten Assad's downfall because neighboring countries, wanting to tamp down on the flood of pills, abandoned him.

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The pill is the synthetic stimulant fenethylline or fenetylline, known by the brand name Captagon, that overtook the Middle East as a popular drug. According to the United Nations Office on Drugs and Crime's World Drug Report last year, "the main departing area for 'captagon' shipments" continues to be Syria and Lebanon, and, "[a]ssuming that all amphetamine seizures reported in the subregion are of 'captagon,' seizures doubled from 2020, reaching a record high of 86 tons in 2021."

Caroline Rose, who studies the Captagon trade at the Washington-based think tank New Lines Institute, told ABC News the drug has a "licit" history and is incorrectly perceived as not being dangerous, and therefore doesn't garner the stigma of party drugs like cocaine or ecstasy. It's also popular in Muslim countries where alcohol is banned by the Quran, she said.

"It makes you feel invincible," Rose said. "It staves [off] hunger and helps keep you up late at night, and you have taxi drivers, university students, poor people in bread lines, wealthy people who want to lose weight -- you have fighters who are using it because it keeps you up late, it gives you energy and you can survive on one MRE [meals ready to eat] a day."

The world's leader in Captagon trade has been Syria, generating an estimated $10 billion for the country -- and an estimated $2.4 billion a year directly for the Assad regime, according to a 2023 study conducted by the Observatory of Political and Economic Networks, a nonprofit that conducts research on organized crime and corruption in Syria.

One person who has had a very close eye on the Captagon trade from Syria in recent years is Rep. French Hill (R-Ark.), one of dozens of lawmakers who co-sponsored the bipartisan Caesar Syria Civilian Protection Act of 2019, which proposed to place heavy sanctions on Assad and his closest allies. The bill ultimately passed as part of the National Defense Authorization Act for 2020.

Hill subsequently introduced the Captagon Act in 2021, which he told the publication The New Arab in 2022 was designed to "disrupt and dismantle the Assad regime's production and trafficking of the lethal narcotic."

"In my view, the Assad regime turning to narcotics production for its main source of revenue was a sign that the world treating Assad like a pariah was working," Hill told ABC News. "It is clear after last week's events that the rot in Assad's military and finances ran deep."

According to Rose, the booming Captagon trade was a "zombie economy" born out of the stiff sanctions imposed on Syria by the United States and Europe — and it was also lining the pockets of the Assad regime.

"If there ever was a perfect case for a narco-state, I think it was Syria, because you had the state security and political apparatus defending Captagon production and putting out a public narrative that there wasn't Captagon but then using the president's brother, all its security apparatus and the Fourth Armored Division all involved in carrying out the trade," Rose said.

Meanwhile, Turkey and Saudi Arabia grew frustrated with their efforts to normalize relations with Assad, and their borders were being flooded with the drug, according to a recent report by the Carnegie Endowment.

According to Rose, in recent negotiation efforts for normalization, Assad was trying to use the power he wielded over the trade of Captagon as leverage over them, and it backfired.

"In many of my discussions with high-level officials in Gulf countries, they said the Assad regime used it [the Captagon trade] as a leverage tool for normalizations, and that the regime often explicitly acknowledged their connection to the Captagon trade," Rose said. "This was a strategy to garner a transactional deal between Gulf states and Damascus, to incentivize them to pay the regime in return for lessened Captagon flows. And that frustrated Gulf states because it really was not the way they wanted to start normalization discussions."

"They believed that after isolating the regime, sanctions and severed diplomatic relations, the regime should be the actor accepting their demands, not the other way around," she added.

Matthew Zweig, a sanctions expert at the lobbying arm of the think tank Foundation for Defense of Democracies, pointed to another question related to Captagon that may have also ultimately contributed to Assad's downfall.

"The question is whether Assad could even control the trade," Zweig told ABC News. "Or did the trade control him? And the same question can be applied to the transitional government. Can they control the trade or will the trade control them? There is a lot of money."

On Sunday, just hours after the Islamist Hayat Tahrir al-Sham group, or HTS, captured Damascus and took power, its leader Abu Mohammad al-Jolani stood up in front of a crowd of supporters inside the capital's historic Umayyad Mosque and declared: "Syria has become the biggest producer of Captagon on Earth, and today, Syria is going to be purified by the grace of God."

The future remains uncertain when it comes to the powerful forces of a cartel economy driven by billions of dollars, high demand and a fragile transitional government.

The experts, as well as Hill, conveyed hope to ABC News that Syria would be able to pivot toward a better future with a normalized economy.

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