It reached the Peninsula in time to be present during the latter part of the siege of Yorktown; was active at Fair Oaks and during the Seven Days Battles, but suffered its most severe loss at Antietam,[8] where 103 were killed, wounded, or captured, among them Chaplain Dwight, who was in the midst of the fight.

The 66th proceeded through Charlestown, West Virginia, and Snicker's Gap, to Fredericksburg, where it lost 75 in killed, wounded, and missing out of 238 engaged.

After the Mine Run movement the regiment went into winter quarters with the Army of the Potomac and when the spring campaign opened, was assigned to the 4th Brigade of its old division.

In Grant's campaign in the Wilderness the heaviest losses of the 66th were suffered during the first week, but it continued in active service through Cold Harbor, where Col. Morris, commanding the brigade was killed, and the Siege of Petersburg, losing heavily in the first assault on the fortifications.

By the end of the first full year of hard campaigning, the regimented reported possession of 344 Enfield P1853s, 424 Mle 1842, and 62 Prussian Potsdam smoothbore percussion muskets (.71 caliber).