He was promoted from captain to lieutenant colonel, and eventually to colonel, serving as Grant’s military secretary — the officer responsible for managing correspondence, preparing orders, maintaining records, and ensuring that the commanding general’s communications were accurate, timely, and properly drafted.

The role suited Parker’s particular skills.

His engineering background gave him a precise, methodical approach to documentation.

His experience as an interpreter and scribe for Seneca leaders had trained him in the art of rendering complex negotiations into accurate written form.

His command of English was exceptional — precise, clear, and capable of the kind of formal register that military documents required.

He participated in some of the most intense campaigning of the war.