A selection of papers on ethnic diversity and conflict:

"The more ethnic groups in a state, the more likely it will have a high rate of guerrilla and revolutionary warfare. And the more religious groups in a society, the more intense the general violence."
https://www.jstor.org/stable/425106

"This research advances the hypothesis and establishes empirically that interpersonal population diversity has contributed significantly to the emergence, prevalence, recurrence, and severity of intrasocietal conflicts."
https://www.nber.org/system/files/working_papers/w21079/w21079.pdf

"Ethnic heterogeneity explains 55% of the variation in the scale of ethnic conflicts, and the results of regression analysis disclose that the same relationship more or less applies to all 187 countries."
https://www.scirp.org/journal/paperinformation?paperid=48450

"conflict arises when groups are neither well integrated nor well separated. In highly mixed regions, groups either don’t develop strong collective identities or don’t lay claim to public spaces. […] Well-separated groups don’t engage in conflict."
https://web.archive.org/web/20170216222018/http://www.necsi.edu/research/social/syria/syria.pdf

"Using data from U.S. states, I investigate the relationship between ethnic diversity and trust. […] The main channel through which ethnic diversity is hypothesized to affect trust is social conflict."
https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1465-7287.2010.00215.x