Operation Werwolf refers to the actual plan by the Nazi party in 1944-5 of creating a commando force that would operate behind enemy lines as the Allies (France, Poland, United Kingdom/British dominions, the United States and other Allied countries) advanced through Germany. On the HBO original series True Blood, Operation Werwolf is actually run by werewolves under the command of Russell Edgington, utilizing the Wolfsangel symbol. The operation is mentioned in the series' third season, but is not explained in-depth, and is primarily used as a plot device to introduce the twist of werewolves servient to a vampire.
The Wolfsangel is a Nordic symbol associated with magical qualities to either ward off or capture wolves, and was used as the pennant of Operation Werewolf (in both real life and the series). In the series, the werewolves under Russell Edgington's command were branded with the symbol for identification. The Wolfsangel is also used today mostly by neo-Nazi groups in North America and Europe except in countries such as Germany where it is banned under anti-Nazi laws.
In season 3, episode "Beautifully Broken", Eric Northman has a flashback to 1945 in Augsburg, Germany, when Godric and Eric posed as Nazi Schutzstaffel agents in order to track down a pack of werewolves who had been interfering with human wars (World War II). They managed to capture a female werewolf by pinning her to the wall by stabbing her arms with a bayonet. Eric demands the werewolf tell them who her master is, but she refuses unless they give her vampire blood. Godric refuses, stating that "the blood is sacred", but Eric promptly bites himself and allows her to drink from his wrist. Eric then asks her, again "who is your master?", to which she replies "he is one of you". However, the vampire is not identified by the werewolf, strengthened by the vampire blood, frees herself and attacks Eric. Godric promptly kills her by snapping her neck.