Schatz says that might have been his most lasting mark on the county.

There was little hint that the commodore would one day personify the divide between North and South — not even during the 1850s, when he was tasked with using the USS Constitution to suppress the slave trade near Africa.

If the mission posed a conflict for Mayo, who owned 15 slaves himself, he didn't let such a contradiction bother him — not yet, anyway.

"He made it clear in his letters: If he was asked to capture slave ships, that's what he'd do," Lee says. "He believed duty to country mattered above all."

'War against brethren'

In times of deep division, of course, one man's loyalty is another man's treason. A century and a half ago, as the Civil War loomed, the Anne Arundel native changed his tune.

Even today, when relations between right and left can seem strikingly bitter, it's hard to imagine the kind of events unfolding in early 1861.

Seven Southern states had seceded, taking federal military installations and their arsenals into their possession. Four other states were threatening to follow. In Maryland, Lincoln shut down newspapers he considered disloyal, jailed officeholders he thought might vote for secession and placed Annapolis and Baltimore under martial law.

Mayo — like virtually all his neighbors in Anne Arundel — was a Southern sympathizer who saw the president "as little more than a would-be dictator," Lee wrote in "Naval Warrior." In early May, he became one of about 300 Navy officers who "went South" that year — that is, he formally refused to take up arms on behalf of the Union.

He was the oldest, and perhaps the most prominent, to do so, and the letter he sent the president that May was very likely the harshest one Lincoln got.

"For more than half a century it has been the pride of my life to hold office under the Government of the United States," Mayo wrote. "For twenty-five years I have been engaged in active service and have never seen my flag dishonored or the American arms disgraced by defeat. It was the hope of my old age that I might die, as I had lived, an officer in the Navy of a free government. This hope has been taken from me.

"In adopting the policy of coercion, you have denied to millions of freemen the rights of the Constitution. In its stead you have placed the will of a sectional party … As one of the oldest soldiers of America, I protest — in the name of humanity — against this 'war against brethren' … You will, therefore, oblige me by accepting my resignation."

Lincoln granted that favor to more than 100 officers making the same request that year. In Mayo's case, he did no such thing.

The White House returned Mayo's letter with a simple notation on the back: "Dismiss by order of the President," followed by the words "Done May 18, 1861." With the stroke of a pen, the administration had given the 52-year veteran the equivalent of a dishonorable discharge, effective on that date.

What motivated him? Some guess it was the Mayo's vitriolic tone. "Ship's captains have to be very egotistical. When they're good, they can be very good; when they're bad, they can be awful," says Owen Daly of Baltimore, a Mayo descendant by marriage and a Navy veteran himself. "I think he just shot his mouth off." Others guess the administration feared Mayo's influence.

No one knows what day he got the notice or whether he got it at all. Mayo never wrote about it. What is known is that by May 18, he was dead — by gunshot, according to a Navy memorandum Lee turned up in his research.

Exoneration

At first, the means and timing of Mayo's demise mattered little to Thomas Bailliere.

It was 1976, and Bailliere, Mayo's great-great-grandson and a Baltimore stockbroker, had concluded Lincoln had acted arbitrarily against a patriot. He petitioned the Board for Correction of Naval Records to change Mayo's record from "dismissed" to "received an honorable discharge by reason of personal resignation."

The board recommended approval, Bailliere says, but further up the chain, the naval historian's office objected. And so began a bureaucratic chess match between Mayo's family and the Navy that lasted for years and in which, it finally became clear, the family had to come at the problem from another angle.