(covers information from several alternate timelines)
An assault fleet was a grouping of ships assigned to work in concert while engaging enemy forces.
In 2155, an assault fleet from the mirror universe, which included the ISS Enterprise (NX-01), was a group of Starfleet combat ships that attempted to put down a rebellion against the Terran Empire. The entire fleet, except for the ISS Avenger, was destroyed by the rebels. (ENT: "In a Mirror, Darkly", "In a Mirror, Darkly, Part II")
During the closing weeks of the Dominion War in 2375, a Federation, Klingon, and Romulan assault fleet, comprising 312 starships, assembled to stop an invading Dominion and Breen fleet from driving the Federation Alliance out of the Chin'toka system. The assault fleet was unsuccessful in stopping the enemy from retaking the system: 311 ships, including the USS Defiant, were disabled by the Breen's energy dissipators and destroyed. The IKS Ki'tang was the sole survivor. (DS9: "The Changing Face of Evil", "When It Rains...")