Abstract: Terrorists and other transnational extremist groups are responsible for thousands of civilian deaths. In confronting extremists, governments have relied heavily on threats, demands, denunciations, and other forms of verbal conflict. Do these efforts at verbal coercion have any effect on terrorist behavior? This analysis focuses on the Islamic State in Iraq and Syria (ISIS), which continues to be the world’s deadliest terrorist group and was responsible for recent high-profile attacks in Baghdad, Vienna, Kabul, and Russia. We use Bayesian structural vector autoregression models to analyze daily event data on interactions between ISIS and foreign governments for the 2014–2020 pe- riod. We find that verbal conflict initiated by governments not only failed to deter ISIS but in fact increased the frequency of ISIS’s attacks on civilians. Additional empirical analyses, combined with evidence from ISIS’s publications and public statements, sug- gest that this effect resulted from a perceived credibility deficit. Extremists use terror attacks to signal that they have the capabilities and willingness to inflict pain and suf- fering on civilian targets. Government attempts to coerce extremist groups verbally, rather than militarily, reflect an underestimation of the group’s capabilities and resolve. In an effort to solidify their reputations, extremists engage in further violence toward civilians, thus leading to worse humanitarian consequences. We extend the analysis to al-Qaeda in Iraq and Boko Haram and find similar results.