Mo. Rev. Stat. § 84.130
The boards may reappoint all members of the police force who have heretofore, or shall hereafter, resign to enter the military service of the United States during any war in which the United States is engaged, without loss of rank, and with the same effect as if an indefinite leave of absence had been granted such members, upon condition that an application for such reappointment shall be made within ninety days after the applicant's discharge from military service and that the applicant is mentally and physically qualified for police duty. The boards may make such reappointments notwithstanding that it may cause the number of policemen employed to temporarily exceed the number herein authorized. All necessary reductions in rank shall be made to enable the boards to reappoint officers above the rank of patrolmen as aforesaid to the end that the number of such officers shall not exceed that provided for in section 84.150.
§ 84.130, RSMo