BEIRUT -- The crisis over chemical weapons in Syria has underlined a central dilemma for the West as it tries to deal with the country's civil war: the lack of attractive alternatives to President Bashar Assad.
The political opposition, largely operating from exile with little credibility on the ground, has been hobbled by infighting. Inside Syria, rebels are also divided. Fighters linked to the al-Qaida terror network have become increasingly dominant, even as the U.S. and its allies try to strengthen rebels seen as moderates with better training and military equipment.
Rebels and Islamic radicals fighting alongside them have already come to blows in some cases, and their divisions could turn into outright battles without the common enemy of Assad.
"Should Assad one day fall from power, it is extremely unlikely that moderates and hard-liners would come to a long-term agreement because of completely diverging interests," said Charles Lister at IHS Jane's Terrorism and Insurgency Center.
President Barack Obama said early on in the 2 1/2-year-old conflict that Assad lost the right to lead because of the brutal oppression of the uprising against his rule, most chillingly displayed in what Washington contends was an Aug. 21 chemical weapons attack on rebel-held areas that killed hundreds of civilians.
But it's not clear who would replace Assad if he were driven from power, either as a result of U.S. punitive strikes for the suspected poison gas attacks or in eventual political transition talks with the Western-backed opposition.
The end of Assad family rule, which has held Syria's rival ethnic and religious groups together with often brutal force since 1970, could lead to further anarchy and break up the country into enclaves ruled by heavily armed warlords.
"The most likely scenario is the Iraq scenario, complete chaos and banditry," said Syria expert Joshua Landis from the University of Oklahoma, referring to years of sectarian violence in Iraq after the U.S. toppled Saddam Hussein in a military invasion a decade ago.
"The idea that somehow you destroy Assad and there will be another central Syrian state is entirely wishful thinking," he said.
U.S. officials have said airstrikes they are considering are not aimed at toppling Assad, but to punish his government for the alleged chemical attack. Still, critics in the U.S. fear such strikes would drag the Americans deeper into the conflict -- and now Obama is considering a Russian plan to secure Syria's chemical weapons stockpiles as a possible alternative to military strikes.
The Obama administration has said its greater aim is to weaken Assad enough militarily so he'll negotiate a transition deal with the political opposition.
So far, its main avenue for doing so has been to use the CIA to quietly train rebels in Jordan. Under a new proposal being discussed within the administration, the program would be greatly expanded by bringing in U.S. military trainers, U.S. officials told The Associated Press last week, though they underlined that no decision has been made.
Since Syria's conflict began in March 2011, fighting has killed more than 100,000 Syrians and uprooted nearly 6 million, or more than a quarter of the population.
Neither the regime nor the rebels has been able to deliver a decisive blow. Rebels control large areas of countryside in the north, east and south, while the regime is holding most urban centers, particularly in the densely populated west.
Armed rebels roughly fall into three categories.
Jihadis, including hundreds of foreign fighters, number up to 10,000, according to Lister -- though he cautioned that any estimates of numbers on the anti-Assad side are rough estimates at best. Many jihadis have links to al-Qaida, and they follow a strong anti-Western ideology calling for global "holy war."
Islamic militants, or Salafis, who are more concerned with Syria than a transnational jihadi ideology, can muster 20,000 to 35,000 fighters, Lister estimated.
Finally, several tens of thousands of fighters are loosely grouped under the Free Syrian Army, which gets weapons from Gulf countries and political backing from the West, Lister said. However, it's difficult to say how many are truly loyal to FSA chief of staff Gen. Salim Idris.
Local rebel commanders have considerable autonomy. They often decide on the spot whether to cooperate with others in battle, creating ad hoc alliances between FSA groups, jihadis and Salafis.
But there have also been turf battles among rebels, even assassinations, as well as splits within blocs. For example, fighters from Chechnya recently announced that they are waging holy war on their own terms and will no longer take orders from the two dominant jihadist groups, including Jabhat al-Nusra, which is allied to al-Qaida.
Jihadis dominate parts of the north and the east, particularly the eastern city of Raqqa, where they have set up rudimentary local governments and, according to some, a rule of terror.
The Syrian Observatory for Human Rights, a Britain-based anti-regime group monitoring the fighting, said jihadists have summarily executed prisoners and have arrested hundreds of civilians, including anti-Assad activists, for allegedly violating Islamic laws in areas under their control.
Some argue that the status quo of a largely deadlocked war serves U.S. interests because America's enemies can be found on both sides -- Iran and the Lebanese Hezbollah aligned with the Assad regime, and jihadis among the rebels.
If America's enemies are fighting each other, it "is not the problem, it's a solution," said Edward Luttwak of the Center for Strategic and International Studies, a U.S. think tank. The rebels should be helped if Assad seems to be winning, and not helped if they start to win, he said.
In making his case last week for military strikes, U.S. Secretary of State John Kerry played down the jihadi threat and argued that a rapid post-Assad transition could help restore what he described as Syria's secular character.
Kerry told a Senate committee that the size of Jabhat al-Nusra is "lower than former expectations," without giving figures. He said the Syrian political opposition is showing signs it is adhering to a "democratic process and to an all-inclusive, minority-protecting constitution, which will be broad-based and secular with respect to the future of Syria."
However, Rami Abdel-Rahman, head of the Observatory, warned that the U.S. is underestimating the influence of the jihadists.
At the same time, initial Western hopes of a palace coup have failed to materialize.
The absence of mass defections at the top level suggests the war has only helped consolidate the regime, and no apparent successor to Assad has emerged who would also be palatable to the opposition as an interim leader.
Assad's rule rests on a coalition of religious and ethnic minorities, including his Alawites, or followers of an offshoot of Shiite Islam, who fear there would be no life for them in Syria if the rebels, most of them Sunni Muslims, take over.
"The Alawites do really feel that this is their only hope, if they cling to Assad," Landis said.