As President Bashar al-Assad's regime teeters on the brink of collapse, military planners in the Middle East and the United States are projecting worrying scenarios of the regime's disintegration. On Friday, opposition forces battled for control of a Damascus neighborhood on a day described as the deadliest since the fighting first began. After assassinating top security officials, rebels seized four border crossing with Iraq and Turkey but were forced to retreat as government forces struck back Friday afternoon, reports The New York Times. Now foreign governments are planning for what's next and it doesn't look pretty. “The disintegration is not abstract; it is real,” said Israel's defense minister Ehud Barak on Thursday. Here are the threats military leaders are planning for:

A Civil War in Lebanon  Many countries in the region are concerned that the collapse of Syria could result in a civil war in neighboring Lebanon. Mary Fitzgerald at The Irish Times reports from Lebanon that the concern is shared by Lebanese residents as well. "Many Lebanese believe that if Syria’s crisis engulfs their country, the spark will come from here in these densely populated streets," she writes. "The area, whose sectarian tensions date back to the Lebanese civil war, has witnessed a series of Syria-related clashes this year that culminated in fierce fighting in June. At least 25 people were killed as the two sides exchanged fire from rooftops, balconies and street corners." To get a sense of the impact this conflict has had, the UN says up to 30,000 Syrian refugees have flooded into Lebanon since the fighting began. 

Chemical Weapons Unleashed One of the primary fears of Israel's military planners is Syria's large stockpile of chemical weapons getting in the hands of radical groups that hate Israel, reports The New York Times Jodi Rudoren. "Israel has now been confronted with a series of complex calculations. Should it strike Syria’s chemical weapons storehouses, as it did a Syrian nuclear reactor in 2007, or would that strengthen Mr. Assad’s hand by uniting the Arabs?" 

Assad Fires on Rebels with Scud Missiles Danger Rooms' David Axe looks into the likelihood that Assad would use his significant stockpile of ballistic missiles against his own people. "Chances are, Syria possesses at least three types of ballistic missile that can be fitted with chemical warheads, according to Dr. Jeffrey Lewis from the James Martin Center for Nonproliferation Studies in California. These include Scuds and SS-21s acquired from North Korea and, less clearly, Fateh 110s transferred from Iran." There are no easy solutions, however, for the international community if Syria wants to attack its own citizens: 

Short of a full-scale intervention, however, there’s not much the world can do to defend the rebels and Syrian civilians against Assad’s chemical-tipped missiles. Even a strictly aerial campaign might not work: Iraqi Scuds managed to hide from U.S. warplanes in 1991 and again in 2003. “What you would have to do is have people on ground engaging in a Scud hunt,” Lewis says. At the very least, that means large numbers of Special Operations Forces plus the troops to support them.

An International Peace Force May Be Needed  There's also the distinct possibility that a successor group to Assad's regime could be highly unsavory or spark infighting across a number of countries, as Bruce Riedel, a former CIA officer, writes for the Daily Beast. " It could be chaos like the Lebanese civil war in the 1970s. A Sunni military dictator may emerge. The Muslim Brotherhood, which led the 1982 Hama revolt and plays a large role in the current insurrection, may emerge dominant. Almost any conceivable successor regime to Assad’s will likely be hostile to Hizbullah and Iran," he says. That could warrant an international effort to calm tensions. "A peacekeeping force should be primarily but not exclusively Muslim in composition. Saudi Arabia, the United Arab Emirates, and Qatar should pay for it. If Russia and China refuse to support it at the United Nations, then the Arab League should be the sponsor."