Is it not unusual for states to ask each other for assistance. This time, North Dakota has asked for assistance from Montana to keep the peace.

According to the official Press Release from Sheriff Brian Gootkin:

As many of you are aware, there have been protests in North Dakota that have recently turned violent. I have worked with the North Dakota Sheriffs in the past and they have requested our assistance.
The Montana Highway Patrol has already sent troopers there to assist.

I will be taking four Gallatin County deputies to help those Sheriffs keep the peace. We will be there for approximately a week.
We are working with Montana DES and Governor Bullock for mutual aid with the state of North Dakota for reimbursement of expenses.
My decision has nothing to do with the politics of the situation and only to do with trying to help deescalate a volatile situation.

We are peace officers and law enforcement and I am confident that if we had a large incident occur here, they would not hesitate to help.
The Montana Sheriff’s and Peace Officers have sent a request to Montana Sheriffs for assistance and are coordinating sending teams from other Offices and our deployment.

Assistance for North Dakota