Is it halachically permissible for a Jew to call the police on a fellow Jew if a person is threatened with imminent violence in the United States. I know there is an issue with informing on a fellow Jew and I was wondering how this pertains to this.
The sefer "Making It Work" has a full chapter on mesira (chapter 20). In it, R' Ari Wasserman lays outs the guidelines very clearly about what is, and is not, a problem of mesira. I recommend his work highly.
On Pg. 387 he writes:
Rav Yaakov Bloy write that if there is no other way to pretect oneself, it is permitted to call the police without first turning to a beis din under any of the following circumstances: if one is being attacked; if he has reasonable grounds to suspect that he is going ot be attacked; and certainly, if he has received threats against his life or safety. This is true, says Rav Bloy, even if the assailant will be fined or imprisoned as a result (Pischei Choshen, Lichos Nezikin, Chapt. 4, footnote 18)