For the purposes of co-operating with and safeguarding the respective interests of the contributing counties in the proportion of their respective contributions, and in respect of such contributions, the respective boards of county commissioners may enter into such proper arrangements and agreements with the secretary of defense, or any other public authority empowered to act in the premises under any act of congress or of the general assembly of this or the legislative authority of any other state, as is necessary for such purposes; and also with counties and other public authorities, of this or other states, with a view to harmonious and efficient action and proportionate contribution, as nearly as may be arrived at or found to be practicable.