Here is the next few letters in the sequence.
P is for: General John Pope
General Pope commanded the Union Army of the Potomac June 26 to September 1862. During his time as commander the battle of 2nd Mannassas took place
Q is for: William Quantrill
Quantrill was an infamous raider who sacked Lawrence Kansas, killing 150 men and boys.
R is for: General John Reynolds
General John Reynolds was one of the best officers the Union Army ever had in their service. His military career was cut short at the battle of Gettysburg where he was shot and killed.