In order equitably to allocate responsibilities between this and adjoining states for the security of interstate bridges and other interstate structures and facilities, in time of war or military emergency or when hostile destructive acts on the part of enemy agents have occurred, are anticipated, or are suspected, the governor is hereby authorized to negotiate and to enter into formal agreements with the governors of the commonwealth of Massachusetts and of the states of Maine and Vermont relative to the protection of such interstate bridges, structures and facilities, provided such other states are authorized to enter into similar defensive agreements. Such agreements shall set forth the specific interstate bridges, structures or facilities for which each state is to provide military protection, if required by war or military emergency, or if requested under such circumstances by the appropriate authorities of the armed forces of the United States. The agreements may authorize the entrance into and the continued presence within this state of the military forces of such other states whenever and to such extent as may be required to carry out the purposes of this subdivision. A copy of each such interstate agreement shall be furnished by the adjutant general to such persons as he may deem necessary.