Pay you earn in a combat zone is exempt from Montana income tax. That includes combat zone pay received by officers who did not get a complete federal tax exemption, and pay you receive while hospitalized as a result of service in a combat zone.
There is nothing to apply for. Leave your combat zone pay out of the income you report on your Montana return, and attach verification of your military status, such as a copy of your orders.
Your orders or Leave and Earnings Statements showing the combat zone service dates, and documentation of any hospitalization resulting from that service.
