The Sierra Defense Fleet was founded when Starfleet completed the construction of the asteroid detection outposts along the Romulan Neutral zone in 2266. The SDF was intended as the first response fleet should the Romulans ever cross the neutral zone and war be declared. Over the years the SDF has taken part in many noteworthy actions in the region, including forming the armada led by Jean Luc Picard to stop Romulan intervention during the Klingon civil war in 2368 and forming Battle Group Omega which was deployed to support the Enterprise E in 2379 when Praetor Shinzon’s plot was uncovered.

In the late 24th century the dynamic of the Sierra sector began to change a great deal as the Romulan Empire recovered from the actions of Reman Praetor Shinzon. As a result, the Sierra Defense Fleet was engaged less as a battle formation and more as a diplomatic and humanitarian armada. During this time the fleet maintained its title and tradition purely through the efforts of its members who lobbied command to keep the formation intact. Even though Starfleet temporarily granted this request, in 2385 the Sierra Defense Fleet was officially listed as decommissioned. Several Admirals working quietly behind the scenes and outside of traditional Starfleet regulations endeavored to keep the fleet intact and operating under it's traditional moniker. Though not a rogue operation, the SDF was no longer directly under the control of Starfleet, and was granted certain latitudes in order to guarantee the security of the galaxy at large. Instead of being assigned ships by Starfleet command, the SDF now recruited their members from the best and brightest officers available, accepting only those who both understood the nature of the assignment and could handle operating outside of Starfleets direct influence.

When the Hobus star went supernova in 2387 the region was once again thrown into turmoil. With the Romulan government in disarray their military commanders began acting on their own authority and the security of the region was once again jeopardized. As a result, Starfleet command sought a means to bring stability to the sector once more. It was at this time they became aware  the SDF was still in operation, without sanction. Unable to either deploy an entirely new fleet to the area or exert control over the SDF in any official manner, Starfleet contacted the Fleet Command to broker an agreement. The SDF would once again enjoy the support of Starfleet, provided they were willing to accept mission assignments from Starfleet Command. This agreement was accepted on the terms that the SDF would not be subject to direct oversight from Starfleet Command, nor the council.

The Sierra Defense Fleet continues to operate with this mandate while working with Starfleet command and other military and civilian organizations in an attempt to bring stability to the area once again. Although primarily a military formation, the Sierra Defense Fleet has adopted the traditional Starfleet goal of exploration and scientific discovery while not engaged in security activities throughout the galaxy.