Mont. Code Ann. § 7-2-4402

Whenever any land contiguous to a municipality is owned by the United States or by the state of Montana or by any agency, instrumentality, or political subdivision of either or whenever any of the foregoing have a beneficial interest in any land contiguous to a municipality, such land may be incorporated and included in the municipality to which it is contiguous and may be annexed thereto and made a part thereof in the manner provided in this part.