After the assassination of President Abraham Lincoln in April 1865, Foster was selected later that year to serve as a member of the Military Commission established to try the conspirators accused of the murder.

The couple had two children together, Clarence Foster (who later lived in Chicago) and a daughter, who died in September 1898, from an accident.

[2] He was quickly promoted to captain in Lew Wallace's 11th Indiana Infantry Regiment and saw action at the Battle of Rich Mountain in western Virginia.

When Confederate General James Longstreet threatened Suffolk in 1863, Peck's garrison was increased to the size of three divisions.

During the subsequent Siege of Suffolk, Foster's brigade manned the southwest front of the city's defenses.

Foster commanded the 1st Brigade of the Union forces on Folly Island during the siege of Charleston Harbor.

Foster notably led his division in an assault on Fort Gregg during the Union breakthrough at Petersburg.

Foster's division was part of the Union force that blocked Robert E. Lee's line of retreat at Appomattox Court House, leading to the Confederate surrender there.