His interests as a young man were chiefly religious, legal and historical, and these remained with him throughout life, his career as a lawyer being conspicuous and successful.

He ceaselessly assisted with business and personal help and friendship the officers and men of the 20th Massachusetts regiment, in which his brother, Henry Ropes, was killed in action at Gettysburg, and after the war he devoted himself to the collection and elucidation of all obtainable evidence as to its incidents and events.

In this work his clear and unprejudiced legal mind enabled him to sift the truth from the innumerable public and private controversies, and the ill-informed allotment of praise and blame by the popular historians and biographers.

Although practically every member of his society except himself had fought through the war, and many, such as Hancock and W. F. Smith, were general officers of great distinction, it was from first to last maintained and guided by Ropes, who presented to it his military library and his collection of prints and medals.

The Army under Pope is a detailed narration of the Virginia campaign of August–September 1862, which played a great part in reversing contemporary judgment on the events of those operations, notably as regards the unjustly-condemned General Fitz John Porter.