The I Corps was created on March 3, 1862, when President Abraham Lincoln ordered the creation of a five-corps army, then under the command of Major General George B. McClellan.

[3] On August 26, the “Pennsylvania Reserves” were transferred to the III Corps of the Army of Virginia and fought at the Second Battle of Bull Run.

[4] In early September, the I Corps, now under the command of Major General Joseph Hooker, followed Lee through Maryland and fought at South Mountain and Antietam.

John Reynolds (who had been elevated to division command of the Reserves) was temporarily detached to train militia troops in his home state of Pennsylvania and did not participate in the Maryland Campaign.

An influx of new volunteer regiments (both three year and nine month) arrived to replenish its ranks, and by November it was back up to full strength.

It was forced to retreat through the town of Gettysburg, taking up defensive positions on Cemetery Hill after the 16th Maine's brave stand of which only 39 soldiers returned.

Newton led it through the remainder of the battle, including the defense against Pickett's Charge, and through the Mine Run Campaign that fall.