The challenge of integrating Muslim immigrants in Western societies has become salient, as evidenced by burkha bans in France and Belgium, cartoon protests in Denmark, and homegrown terrorism in the United States, despite significant differences in the political institutions and public policies of host countries. The question is, if different institutions and public policies dont matter (at least to the extent that they were expected), what else matters in integration? This study theorizes that host countries violent conflicts in Muslim countries have a negative impact on the social cohesion of Muslim immigrants and host societies. For the transmission mechanism, I suggest that external violent conflicts are particularly utilized by transnational Islamist networks to strengthen in-group/out-group identities and contribute to the mobilization of these groups against each other. Conducting a quantitative analysis of sixteen Western countries for the period 1970-2010, I find that institutions and public policies have a weak positive impact on social cohesion, whereas the external conflict variable has a strong and negative impact. Employing case studies, I also trace the processes through which external conflicts are transmitted to host countries.