 Complete the following phrase:  Walker was born into a prominent Boston family, the son of the economist and politician Amasa Walker, and he graduated from Amherst College at the age of 20. He received a commission to join the 15th Massachusetts Infantry and quickly rose through the ranks as an assistant adjutant general. Walker fought in the Peninsula Campaign and was wounded at the Battle of Chancellorsville but subsequently participated in the Bristoe, Overland, and 