Mo. Rev. Stat. § 86.580

In all systems or plans heretofore or hereafter established by law or ordinance for the pensioning of the salaried members of the organized police force or fire department and the widows and minor children of deceased members, of any municipality of this state, for all honorably discharged members of the armed services of the United States in any war, who reenter or have reentered the service of any such police force or fire department within one year after such honorable discharge, service in such armed services shall be counted as service on such police force or fire department in determining the right to any pension.