On May 4th the 138th, which is now part of 6th Corps, marches toward the Rapidan River along with the Army of the Potomac. The 67th Pennsylvania Infantry will be attached to the regiment to increase their numbers and Colonel Keifer who was the brigade commander will be replaced by General Truman Seymour. The regiment will reach the day's goal of advancing to Germanna Ford, 18miles from the starting point. They will then cross the Rapidan on a pontoon bridge and camp out for the night on the South side of the river. The next day, May 5th, the Battle of the Wilderness will commence just south of the 138th position. That afternoon the regiment was ordered to the part of the front 6th Corp part of. Heavy fighting had already begun. The brigade that included the 138th will be part of the extreme right flank and take positions in the thick forest. Around dark orders were given for the brigade to attack for only a Confederate skirmish line lay ahead of them. Obeying orders the men advanced through the growing darkness to find not a skirmish line, but a line of entrenched Confederates. A heavy engagement pursued. 

   The brigade was pulled back after a prolonged firefight. The regiment would only take 12-15 casualties as they were the second line in the brigade. During the night the sounds of the Confederates strengthening their earthworks could be heard. On the morning of May 6th the regiment was ordered to attack once more and this time in daylight. The enlisted men who had fought the night before knew the attack would fail. "[It] had no promises of victory, for every man who bore his part in it, from the private soldier up to the Regimental commander, knew by the difficulties already met, that such an attack in such force, was next to madness." said one member of the 138th. To make matters worse the 138th Infantry would be placed at the front of the attack, almost guaranteeing disaster. The regiment slogged through the heavy underbrush all while under fire. The regiment would pay a heavy price trying to assault the earthworks. The impenetrable brush that had hindered their attack was almost completely eradicated by the heavy fire. Total number of casualties for that day would number 153.
 
      That night the unit to the right of the 138th was overrun in a surprise Confederate counterattack. Before the men had time to grab their weapons and form a battle line a mob of retreating Federal soldiers and behind them sprinting Confederates where upon them. The 138th quickly retreated. The rebels took man prisoners as a result, including General Seymour. However, by nightfall the line would be restored. The retreat would signal the end of the fight for the 138th Pennsylvania Infantry. The next day May7th, the unit would march toward Spotsylvania.