Ring of Fire — John Winch

A former Confederate spy arrives in San Francisco in June 1866, with the intention of re-igniting the war. He revives a group of pro-Confederate conspirators to help him carry out his plan of shelling the city from a ship outfitted with cannon, which is to be the signal for the thousands of disgruntled Southern men to take up arms again. However, the murder that opens the narrative leads to an investigation. Soon the U.S. Army lieutenant assigned to the case uncovers the conspiracy, which begins a race to stop the conspirators.