Anyone who is at all interested in democratic development ought to find a bookstore that stocks the Journal of Democracy. I have yet to pick up a copy that didn't have numerous articles worth reading. The latest issue has an excellent piece by Sheri Berman (I can't link to the full text; as an aside, I really hope this open model of academic publishing catches on) titled "Taming Extremist Parties: Lessons from Europe" in which she chronicles the political history of Western European communist parties from the end of World War I through the fall of the Soviet Union. She contrasts the generally radical, "antisystem" behavior of these parties during the interwar years (behavior which, paradoxically, contributed to the collapse of democracy and the rise of fascism in much of Western Europe) with the more democratically engaged communist movements of the postwar era.
She attempts to find some common ground between "optimists" who believe that democratic participation has a tendency, through a variety of mechanisms, to de-radicalize and "tame" extremist parties, rendering them less threatening to the democratic system as a whole, and "pessimists" who believe that extremist parties merely use elections as a means to an end without ever truly subordinating their agendas to democratic processes, pursuing a policy of "one man, one vote, one time." Obviously, this issue is germane to today's Middle East, where the question of how Islamist parties ought to be treated by local governments and the international community as a whole remains hotly contested. It is also relevant to places like India, which continues to experience challenges from radical movements that operate both within and without democratic institutions.
In a nutshell (and this is but the roughest sketch of her argument), Berman believes that when democratic institutions are reasonably strong, the mechanisms that the optimists believe will moderate extremist parties do just that (briefly, they identify a "Downsian" phenomenon whereby radical movements must moderate their positions in order to garner a reasonable plurality of votes, a "Michelsian" phenomenon which posits a moderating influence of bureaucratic structures that are necessary in order to compete in elections, and a "pothole theory" of democracy whereby radical programs are sidetracked by the more mundane necessities of day-to-day governance). When state and democratic institutions are weak and have shallow roots in the populace at large, however, extremist parties face fewer pressures to moderate their tactics and goals, and may use elections and an open civil society to foment discord and promote more radical agendas.
Berman notes that the dark days that followed the First World War, when Western Europe was facing unprecedented economic and political crises and Russian Bolshevism actively supported worldwide revolution through the Comintern, were ripe environments for extremist movements. Support for communist movements at the time "varied... in inverse proportion to the health of a country's democratic regime." In Germany, France and Italy, communist movements garnered significant support (though nowhere near a majority of the populace), and had few real incentives to accept the ultimate legitimacy of democracy. They ran slates in elections, but also engaged in violent, destabilizing activity that the weak governments and state institutions at the time could not tamp down. They followed accepted Leninist tactics, expelling moderates from their parties and undermining the governments in which they sometimes half-heartedly participated. In short, they remained radical in their aims as well as in their tactics. It was in part their unwillingness to moderate and ally with the centrist coalitions that held Western European democracies together that fomented fascist transformations leading up to the Second World War.
Her reluctance to tackle the critical issue - why democracy succeeded after the War where it had failed before it - is understandable. Full answers to such questions are the stuff of doctoral dissertations (one day perhaps), but they are critical to an ability to apply the European cases to contemporary regimes, so I'll at least offer a few thoughts. First, in the immediate aftermath of War, when democratic regimes were at their weakest, the presence of several million Allied troops on the European continent certainly helped stabilize things. The prospect of prolonged military occupation, for example, persuaded the French Communists (who had what amounted to an army and were the most coherent organization in postwar France) to use the ballot box, rather than the streets, as their route to power. These troops, in effect, permitted state building on steroids, giving reconstructed democratic governments the ability to quickly achieve Weber's "monopoly of violence" over their territories without too much struggle.