Jim,

Here are a few things to get you started. I presume your ancestor was the Lewis Hamm in the 25th Texas Cavalry (CS)? Are you aware that he applied for and received a Union Veteran's Pension from the State of Indiana? If you'd like any of these records, please click on the link below or at the top of this page.

Eighteenth Infantry INDIANA
(3 years)

Eighteenth Infantry. Cols., Thomas Patterson, Henry D.
Washburn Lieut.-Cols., Henry D. Washburn, DeWitt C. Thomas,
Jesse L. Holman William S. Charles, James C. Black, Josiah
Campbell; Majs., DeWitt C. Thomas, Jesse L. Holman, John C.
Jenks, Jonathan H. Williams, James C, Black, Napoleon H.
Daniels.

This regiment was organized at Indianapolis, and was mustered
in on Aug. 16, 1861, for three years. It left the state the next
day for St. Louis and accompanied Fremont into Missouri.

On its return it moved with Pope's army to the Blackwater and
aided in the capture of a large number of prisoners. In Feb.
1862, it marched to Cross Hollow, Ark., and in an engagement
near Leesville in March its brigade saved another from
capture, the 18th recapturing the guns of the Peoria
artillery. The regiment participated in the advance at
Elkhorn Tavern, when the enemy was forced from the field, and
then marched for Helena, Ark., being engaged at Cotton Plant
early in July and reaching Helena on the 13th.

On Oct. 11, it moved for southeastern Missouri, where it
passed the winter, and was transferred to Grant's army in the
spring of 1863, participating in the engagement at Grand Gulf.
At Port Gibson it captured a stand of colors and some
artillery; was engaged at Champion's Hill, Black River bridge,
and at Vicksburg from May 19 until its fall, being in the
assault on the enemy's works and the first to carry its colors
to the parapet.

It was in the Bayou Teche campaign and other operations in
Louisiana during the fall, and on Nov. 12 embarked for Texas.
It was engaged at Mustang Island, and in the attack on Fort
Esperanza. It reenlisted at Indianola in Jan. 1864 and was
furloughed home, stopping at Baton Rouge to aid in repelling a
force about to attack the garrison there.

It was ordered to Virginia in July, joined Gen. Butler's
forces at Burmuda Hundred, and was engaged in several severe
skirmishes at Deep Bottom. It was then transferred to
Washington and assigned to the 2nd division 19th corps, which
joined Sheridan's army in Virginia.

It participated in the battle of the Opequan losing 54 killed
and wounded; aided in the defeat of Early at Fisher's Hill,
fought at Cedar Creek, where it lost 51 killed and wounded and
35 prisoners.

Took transports for Savannah GA, Jan. 6, 1865, and was engaged
for three months in building fortifications. It was detached
May 3, and sent to Augusta, GA, raising the Stars and Stripes
over the arsenal for the first time since the beginning of the
war. It returned to Savannah on June 7, was sent to the
southern part of the state, and was mustered out Aug. 28,
1865.

Its original strength was 1,056. Gain by recruits, 140;
reenlistments, 359; total, 1,555. Loss by death, 180;
desertion, 53; unaccounted for, 156.

Source: The Union Army, vol. 3