What to know after Syrian rebels force Bashar al-Assad from power ...

Syria's long-running civil war came to a head Sunday when opposition forces entered the capital city of Damascus and the government of President Bashar al-Assad collapsed. Russian state media later reported that Assad and his family had fled to Moscow.

Crowds gathered in Damascus to celebrate the fall of Assad's government with chants, prayers and occasional gunfire, marking the end of a regime that, between the ousted president and his father, had ruled over Syria for half of a century.

"At long last, the Assad regime has fallen," President Biden said Sunday afternoon at the White House after convening his national security team to discuss the developments. He said the fall of Assad presented an "historic moment of opportunity," and pledged support for Syria and its neighbors against any threats.

Syrian Prime Minister Mohammed Ghazi Jalali said early Sunday that he'd remained in Damascus and the government was ready to "extend its hand" to the opposition and hand over its functions to a transitional government, The Associated Press reported.

What happened in Syria's civil war?

The war in Syria began in 2011 when a pro-democracy uprising calling for the end of Assad's long reign escalated quickly into a brutal civil war. Since then, the conflict has claimed more than 500,000 lives and displaced some 12 million people from their homes across the country.

After a decade of brutal conflict, the war largely stalled after the Assad regime regained control over many major cities — with vital support from Russia and Iran, in addition to the powerful Iranian proxy group Hezbollah in Lebanon. Some northern and eastern areas of the country remained under the control of Kurdish-led, U.S.-backed rebel groups.

But in late November, rebel forces in the northwest launched a shock offensive, led largely by the militant group Hayat Tahrir al-Shams (HTS). HTS has tried for a number of years to publicly distance itself from its jihadist roots as a regional branch of al Qaeda. With other conflicts currently occupying Assad's backers Iran, Russia and Hezbollah, the rebel forces are believed to have seen an opportunity to renew their fight against his army.

The leader of HTS told CNN in a recent interview that the goal of the offensive was to remove Assad from power.

"When we talk about objectives, the goal of the revolution remains the overthrow of this regime," Abu Mohammad al-Golani told CNN. "The seeds of the regime's defeat have always been within it… the Iranians attempted to revive the regime, buying it time, and later the Russians also tried to prop it up. But the truth remains: this regime is dead."

How was the Assad regime toppled so quickly?

"Clearly HTS's military and political skills, honed over four years in rebel-held Idlib since the last ceasefire, have been a major factor in their successes. But the weakness of Assad's forces has been the other side of the story," Christopher Phillips, an associate fellow with the Chatham's House think tank's Middle East program, said in an analysis.

Phillips said international factors played a significant role in Assad's inability to maintain power, including Assad's closest allies being weakened or otherwise occupied, but he said there were also a number of important domestic factors.

"Assad has been complacent. His military has melted away because it is deeply demoralized. Assad has focused on winning international recognition rather than consolidating his 'victory' in the civil war at home," Phillips said. "There has been no peace dividend for those who stayed loyal during the war, or for those former rebel areas that opted to reconcile rather than fight the regime. Instead, regime corruption and continuing sanctions, not to mention the regime's characteristic brutality, has meant life for Syrians living under Assad has been grim. In retrospect, it is unsurprising then that, after HTS' advances in the north, former rebellious parts of Homs, the Houran and Damascus rejoined the rebel cause this week, ultimately hastening the regime's collapse."

U.S. and Israel carry out strikes in Syria

The United States has around 900 soldiers on the ground in Syria. The U.S. forces have operated in the country for years as part of the American effort to support the Kurdish-led forces, and to combat the ISIS extremists who seized control of a huge part of Syria and neigboring Iraq in the years after the war began. 

The U.S. said it carried out airstrikes against ISIS operatives and camps on Sunday, hitting 75 targets.

U.S. Secretary of Defense Lloyd Austin told journalists during a visit to Japan on Sunday that as the situation in Syria unfolds, "there's a potential that elements in the area, such as ISIS, to try to take advantage of this, this opportunity, and regain capability." 

Austin said the U.S. was "still evaluating the results" of the strikes but that he believed they were successful in the goal "to keep the pressure on ISIS and to deny them the ability to easily resurge."

Israel also carried out airstrikes Sunday in Syria, hitting sites linked to Syrian state forces and Iran, the Reuters news agency reported. Israel has moved tanks into a demilitarized zone along the country's border with Syria, but the Israeli military said it intended to remain out of any conflict there, calling the move a bid to prevent any possible spill-over of fighting into Israel, according to Reuters.

The Associated Press contributed to this report.