Alex Downes, who has just joined the department at GWU, has a great piece on this topic, with this title, in the new Boston Review. Key paragraph:
Is the bloody aftermath of regime change in Afghanistan and Iraq the exception or the rule? Does regime change work?
The short answer is: rarely. The reasons for consistent failure are straightforward. Regime change often produces violence because it inevitably privileges some individuals or groups and alienates others. Intervening forces seek to install their preferred leadership but usually have little knowledge of the politics of the target country or of the backlash their preference is likely to engender. Moreover, interveners often lack the will or commitment to remain indefinitely in the face of violent resistance, which encourages opponents to keep fighting. Regime change generally fails to promote democracy because installing pliable dictators is in the intervener’s interest and because many target states lack the necessary preconditions for democracy.
The rest of the piece is a summary of political science’s findings on the (usually dismal) record of efforts by outside actors to change regimes.