Montana Code Annotated § 46-6-311(2)(b), Arrest of Predominant Aggressor
(1) A peace officer may arrest a person when a warrant has not been issued if the officer has probable cause to believe that the person is committing an offense or that the person has committed an offense and existing circumstances require immediate arrest.
(2)(a) The summoning of a peace officer to a place of residence by a partner or family member constitutes an exigent circumstance for making an arrest. Arrest is the preferred response in partner or family member assault cases involving injury to the victim, use or threatened use of a weapon, violation of a restraining order, or other imminent danger to the victim.
(b) When a peace officer responds to a partner or family member assault complaint and if it appears that the parties were involved in mutual aggression, the officer shall evaluate the situation to determine who is the predominant aggressor. If, based on the officer’s evaluation, the officer determines that one person is the predominant aggressor, the officer may arrest only the predominant aggressor. A determination of who the predominant aggressor is must be based on but is not limited to the following considerations, regardless of who was the first aggressor:
(i) the prior history of violence between the partners or family members, if information about the prior history is available to the officer;
(ii) the relative severity of injuries received by each person;
(iii) whether an act of or threat of violence was taken in self-defense;
(iv) the relative sizes and apparent strength of each person;
(v) the apparent fear or lack of fear between the partners or family members; and
(vi) statements made by witnesses.
(3) If a judge has issued a standing order as provided in 45-5-209, a peace officer shall give a defendant charged with or arrested for partner or family member assault or a violation of 45-5-202 or 45-5-213, if the victim is a partner or family member of the defendant, both written and verbal notice of the no contact order issued pursuant to 45-5-209. The notice must include specific conditions as ordered by the court.