Meredith and his Hoosiers fought their first engagement during the Northern Virginia Campaign at Brawner's Farm, where his horse was shot from under him, crushing him and breaking several ribs. During the Maryland Campaign, Meredith took part in the Battle of South Mountain, but then abruptly reported himself unfit for duty due to the lingering effects of his injuries at Brawner's Farm and fatigue resulting from to the long march up from Virginia. He went to Washington to rest and recuperate while his replacement, Lt. Col. Alois O. Bachman, was killed while leading a charge near the Cornfield at Antietam. As far as Brig. Gen.John Gibbon was concerned, he deserved to be stripped of his command for this. A month later, Gibbon was promoted to Maj. Gen. and given the 2nd Division, I Corps to command, then recommending either Colonel Lysander Cutler of the 6th Wisconsin or Colonel Lucius Fairchild of the 2nd Wisconsin to take over the Iron Brigade. However, Major General Joseph Hooker (who had commanded the I Corps at Antietam and was recovering from a wound sustained in that battle) was visited by Meredith requesting promotion to brigadier general. Regardless of the Antietam fiasco and Gibbon's disdain for him, he had powerful political connections in the form of Indiana Governor Oliver Morton. This was enough to convince Hooker, who submitted his request to Washington. In November, Meredith thus took the field wearing the stars of a brigadier general while John Gibbon fumed and cursed Hooker as a man who had sacrificed his principles for political gain. Meredith led the brigade in combat for the first time at Fredericksburg, where he drew the ire of division commander Maj. Gen. Abner Doubleday, who temporarily replaced Meredith with Col. Lysander Cutler.