“This is the moment of truth for Bashar al-Assad,” said Jean-Pierre Filiu, a visiting professor at Columbia University who has written extensively on Syria. “He has potentially the ability to impose reforms on his own Baath Party, but has he the will to do so?”

The consequences of his decision could be momentous, perhaps more so than in any of the other revolts yet seen in the Middle East. Unlike Egypt and Tunisia, Syria is home to a checkerboard of defensive religious and ethnic minorities, and many fear that the end of the Assad family’s 40-year dynasty could unleash brutal revenge killings and struggles for power. The chaos could easily spill over Syria’s borders, to neighboring Lebanon and beyond.

The Obama administration has already accused Iran of helping to prop up Mr. Assad. If Syria fell, it would mark a striking setback for the theocratic regime in Tehran, which has depended on Syria for its influence in the Israeli-Palestinian conflict and elsewhere. Yet Iran’s nemeses — including Israel, the United States and Saudi Arabia — are also deeply unsettled by the prospect of regime change in Syria, which could set off a messy Iraq-style civil conflict.

Even if Mr. Assad survives, the turmoil is likely to have profound effects on Middle Eastern politics, some analysts say. “Our entire Syria policy for the past two and a half years has been based on getting Syria and Israel back to the peace table,” said Andrew Tabler, an analyst with the Washington Institute for Near East Policy. “Now that Bashar has accused Israel and the United States of fomenting this challenge to him, it will be even harder for him to do that.”

In a sense, the crisis Mr. Assad now faces is the same one that has defined his years in power: Again and again, he has inspired hopes, both at home and abroad, only to disappoint them. Western leaders courted him, in hopes he would democratize his country, make peace with Israel and stop supporting the militant groups Hamas and Hezbollah. Syrian liberals enjoyed a brief “Damascus Spring” of greater openness after his accession, but it soon faded. His personal style helped foster those illusions. Unlike his stern father, Hafez al-Assad, who took power in a coup in 1970, Bashar al-Assad seemed quiet and almost meek. He had studied ophthalmology in London, and had an elegant British-born wife. He speaks fluent English and French, and reads widely.