The attack on Deep Space 9 was by three Jem'Hadar fighters, who emerged from the Bajoran wormhole and destroyed the USS Aldebaran which was defending DS9 while it and it's usual support vessel the USS Defiant was under going a major refit program. The Defiant was able to put up some fight, and with the aid of a forth Jem'Hadar fighter that inexplicable sided with the Federation vessel the attack was quashed. But DS9 still took heavy damage and the Alliance perceived the attack as a possible first strike of a renewed Dominion offensive following the only recently concluded Dominion War. The Alliance convened a council and agreed to form the task force to enter the Gamma Quadrant and meet any new Dominion threat head on.
The force rendezvoused in the Gentarist system, an area about a day and half away from DS9 at high warp. The Federation portion of the fleet, led by the USS Cerberus under Admiral William Ross was the last portion of the force to leave the system as it waited for the USS Enterprise to arrive. The fleet was forced to leave before the Enterprise made it there to catch up with the rest of the armada. Though the Enterprise, which had been damaged in the Badlands, made it's was to DS9 to receive repairs, at the time unaware of the current events, and was awaiting the task force there when they arrived.
Shortly before the task force arrived further Jem'Hadar attempts on DS9 resulted in the destruction of the station's fusion core, though thanks to the help of the Jem'Hadar Taran'atar not the entire station. Following the incident Taran'atar explained the situation; the attacks were by a rogue faction of Jem'Hadar, led by Kitana'klan, and did not represent Dominion interests.
When the task force got to DS9 representatives from the allies - Admiral Ross, Captain Picard and Commander Vaughn from Starfleet, Colonel Kira of the Bajoran Militia and commander of Deep Space 9, Captains Klag and R'taga of the Klingon Empire and Commander Sartai of the Romulan Star Empire - meet to hear what Taran'atar had to say. He was able to convince the leaders that the Dominion had no intension of further hostilities and a short time after the task force cancelled its plans to enter the Gamma Quadrant and stood down. (DS9 novels: Avatar, Book One, Avatar, Book Two)