Max Boot, Jeane J. Kirkpatrick Senior Fellow for National Security Studies
Actually, arming rebel groups has had a pretty good record of success. In fact, as I point out in my book, Invisible Armies: An Epic History of Guerrilla Warfare from Ancient Times to the Present, there is no more consistent determinant of the success or failure of any insurgency than the degree to which it receives outside support.
The practice of powerful states arming rebels groups abroad is not a new development—France armed the Americans who rebelled against the British Empire in 1775 and Britain armed the Spaniards who revolted against French rule in 1808. The Soviets and Chinese armed the Vietcong and the North Vietnamese army in the 1960s and they succeeded in defeating the United States. The United States returned the favor in Afghanistan, arming the mujahedeen who defeated the Red Army in the 1980s.
Providing arms is only one form of support. A state can also provide actual armies or air forces to fight alongside rebel groups, as well as safe havens (of the kind which the Taliban now enjoy in Pakistan and the Revolutionary Armed Forces of Columbia or FARC enjoy in Venezuela ) to support them. The United States has been particularly effective when it has provided not only arms but airpower to help insurgents—as in Kosovo in 1999, Afghanistan in 2001, and in Libya in 2011. But while it is not hard to overthrow an existing regime by providing assistance to an insurgency, it is much harder to establish security and stability afterwards. Libya serves as a present example, and this would certainly be true in post-Assad Syria.