The Commodore, or Commodore Hornblower (as it was published in the US) was published in 1945. It was the fourth 'Hornblower' book to be written, but in terms of the internal chronology, it is the ninth book.
Captain Sir Horatio Hornblower has married Lady Barbara Leighton (née Wellesley) and is preparing to settle down in Kent. He misses the sea and quickly accepts an opportunity to command a squadron in the Baltic. The mission is both diplomatic and military in nature, the primary goal is to bring Russia into the war against Napoleon.
As is usual for the series, Hornblower must overcome the difficulties of duty and command. In Commodore Hornblower, the most memorable ship-ship battle includes the use of mortars.
Russia agrees to enter the war after Hornblower meets with officials (including the Tsar) at Kronstadt. Here, Hornblower meets with a Russian countress - and while the actual events are not portrayed, it is obvious to the reader that such an encounter has taken place.
Hornblower's men then play a crucial role in the defence of Riga, Latvia, which is under siege by the French. Again the mortars, and also amphibious tactics, are important to the military operations. Historically it is not known whether any British Naval forces were present during the siege of Riga.
The seige is broken and Hornblower chases the French Army on horseback, only to fall seriously ill with typhus.