The NY Times has an article "Give peaceful resistence a chance". It claims that peaceful resistance has a much better chance of succeeding than armed resistance.
Consider the Philippines. Although insurgencies attempted to overthrow Ferdinand Marcos during the 1970s and 1980s, they failed to attract broad support. When the regime did fall in 1986, it was at the hands of the People Power movement, a nonviolent pro-democracy campaign that boasted more than two million followers, including laborers, youth activists and Catholic clergy.
Indeed, a study ("Why civil resistance works") I recently conducted with Maria J. Stephan, now a strategic planner at the State Department, compared the outcomes of hundreds of violent insurgencies with those of major nonviolent resistance campaigns from 1900 to 2006; we found that over 50 percent of the nonviolent movements succeeded, compared with about 25 percent of the violent insurgencies.
According to the article peaceful resistance attracts more people and as a consequence police and army men are more likely to have relatives among them. Also when the police or army is faced with armed resistance that creates solidarity amongst them against the protesters. This is also the reason Mubarak sent thugs to provoke the population to violence that he could crush. But where Mubarak failed Gadhaffi's opponents soon resorted to armed resistance - decreasing their chance of success.
There is one weak point in this reasoning: It leaves the question whether peaceful resistance is possible against ruthless totalitarians like the regime in North Korea that never have been shy about using violence.
In the article Social science and the Libyan adventure Foreign Policy listed social research that shows that foreign interventions seldom lead to more democracy.
On 24 august 2011 FP had an article "Think Again: Nonviolent Resistance" by Erica Chenoweth that explains the same logic.