The Horse Soldiers

In 1863, a Union outfit is sent behind Confederate lines in Mississippi to destroy enemy railroads but a captive southern belle and the unit's doctor cause frictions within ranks.
A Union Cavalry outfit is sent behind confederate lines in strength to destroy a rail supply center. Along with them is sent a doctor who causes instant antipathy between him and the commander. The secret plan for the mission is overheard by a southern belle who must be taken along to assure her silence.