Walter took command of the USS Assurance in the early 2350s. In 2362, Walter lead a mission with Captain James Leyton and the crew of the USS Okinawa against the Tzenkethi in the Entelior star system. After the Assurance was disabled by a Tzenkethi attack and was caught in the gravitational pull of Entelior IV, Benjamin Sisko was able to use the Okinawa shuttlecraft Chatan, Nago and Naha to slow the descent of the starship. Soon after, a Tzenkethi starship abducted Captain Walter and his crew. Walter was tortured by the Tzenkethi and suffered a burn on his forehead when a Tzenkethi touched it. Dermal regenerators were unable to repair the scarring.
By 2381, Walter had been promoted to Admiral. When the Borg began their invasion in that year, Admiral Walter visited Sisko on Bajor and asked him to rejoin Starfleet to fight against them. Following the end of the Borg threat, Walter asked Sisko to remain in Starfleet, citing his importance to the rebuilding of the Federation. (ST - Typhon Pact novel: Rough Beasts of Empire)