When and why do “radical flank” groups employing extreme means—such as Hamas, the Industrial Workers of the World, and New York Radical Women—help or hurt the success of their broader political and social movements? We argue that weak flanks are more likely to make their movement’s center seem more moderate by comparison and generate effective good cop/bad cop dynamics, while strong flanks are more likely to hurt the credibility of the movement and muddle its messaging. Weak flanks are more likely to quietly shift the status quo from one of compellence to deterrence, while strong flanks are more likely to inspire a backlash while denying the center plausible deniability. Movements with weak flanks are therefore more likely to succeed, while those with strong flanks are more likely to fail. In order to analyze the behavior and effectiveness of flanks, we will focus on the efforts of two radical flanks—Gush Emunim and the Hilltop Youth—within the Israeli settlement movement to expand into the West Bank in three periods: 1967-1983, 1990-1996, and 1997-present. This longitudinal analysis allows for tight comparisons and analysis of causal mechanisms with key actors and ideologies held constant. We conducted our analysis using our original dataset on “price-tag” incidents—attacks that burn Palestinian mosques and destroy property, accompanied by threatening graffiti that references Israeli settlers, outposts, and anti-Arab slogans —and another dataset on settlements and outposts we revised, coupled with an analysis of the secondary source literature and interviews we conducted with relevant participants and observers. We find that the weak flanks during 1967-1983 and 1997-present helped expand Israeli control of territory in the West Bank, whereas the strong flank during 1990-1996 hurt the reputation, election prospects, and geographical spread of the settlers. This empirical analysis provides not only a plausibility probe of the theory, but also a systematic new explanation of mechanisms and variation in Israeli settlements over time, which have previously been described using ideological and religious explanations. We conclude by presenting implications for scholarship and policy, with a focus on how the strength and dynamics of flank and center are essential to understanding the effectiveness of social movements.