This week, Syrian forces began pounding Aleppo — a rebel-held city about the size of Chicago — forcing more than 200,000 panicked residents to flee, according to the United Nations. On balance, though, the tide of war seems to have shifted in favor of the opposition, following significant territorial gains in recent weeks, the assassination of several top officials in Assad's inner circle, and mounting defections by Syrian servicemen and diplomats. "Momentum is clearly against [Assad]," Michael Hammer, an assistant U.S. secretary of state for public affairs, recently said. "We are confident that his days are numbered — that he is losing his grip on the country." But as eager as many Western observers are to see Assad fall, they're aren't exactly optimistic about Syria's post-Assad future. What would happen if Assad is ousted? Here, four somewhat gloomy scenarios:

1. A security vacuum

U.S. officials are concerned that Assad's demise could lead to a total dissolution of the country's "hated security and government apparatus," say Anne Gearan and Joby Warrick at The Washington Post. Syria might plunge into the type of violence-wracked power vacuum that engulfed Iraq shortly after the U.S. invasion in 2003, when U.S. authorities oversaw a "de-Baathification" of the armed forces, firing even low-level soldiers and police officers that had ties to Saddam Hussein's Baath party. Indeed, the fear of a power vacuum "is a major reason President Obama has all but ruled out direct military help for the rebels."

2. Loose chemical weapons

U.S. and Israeli intelligence officials are taking steps to ensure that Assad's arsenal of "chemical weapons [doesn't] fall into the hands of terrorist groups," says Paul Alster at Fox News. Hezbollah, the Shiite militant group that has long been backed by Assad, "is the prime candidate to take possession of the armaments," but Syria has also become a magnet for several Islamist groups, including al Qaeda, that could take advantage of Assad's demise to seize the weapons.

3. A drawn-out, bloody civil war

Assad's ouster could very well usher in a protracted period of conflict between Syria's various religious and ethnic sects. The current conflict is already divided along sectarian lines, with the country's Alawites and Christians largely supporting Assad, an Alawite himself. The country's majority Sunnis, who have long suffered discrimination under Assad, could unleash an "ethnic backlash" against their former oppressors if Assad falls, says Peter Apps at Reuters. The ensuing chaos could resemble Lebanon's sectarian conflict in the 1980s, which lasted well over a decade.

4. A regional conflict

A drawn-out Syrian civil war could explode outward, spilling "into neighboring Lebanon, Jordan, Iraq, and Turkey," says Vali Nasr at The New York Times. Lebanon and Iraq are similarly divided along Sunni-Shiite lines, and the spread of the Syrian conflict could open Sunni-Shiite fissures across the Middle East. The breakup of Syria "poses a graver threat to the Middle East and to America's long-run interests in the region than does Iran's nuclear program," and the U.S. could begin working with regional players — possibly including Iran, Assad's strongest ally — to "devise a post-Assad power-sharing arrangement that all sides could sign on to."