Battle simulations, also known as war games, battle drills, readiness drills or military exercises were simulated experiences designed to test combat abilities and maneuvers for commonly encountered conflict situations, in order to hone the speed and accuracy skills of the crew.
Major J. Hayes demonstrated to Lieutenant Malcolm Reed the usefulness of computer battle simulations when exploring weaknesses in Xindi-Insectoid starships shields, explaining that "[s]cenarios like this can tell us a lot about the enemy's tactical capabilities. Much more that we'd learn from a standard analysis." Reed saw the simulation as little more than a game, however, when Hayes pointed out that the simulation found a vulnerability, specifically, minimal shielding around the impulse manifolds, they concluded that a pair of torpedoes would be able to take out the engines. Reed later complimented Hayes on his new tactical approach. (ENT: "Hatchery")
During the 23rd and 24th centuries, Starfleet Academy routinely used battle simulations to train its cadets, including the notorious Kobayashi Maru scenario, which detailed a no-win scenario. (Star Trek II: The Wrath of Khan; Star Trek)
When William T. Riker participated in a battle simulation at the Academy, he calculated a sensory blind spot on a Tholian vessel and hid within it. Many years later, in 2365, Lieutenant Commander Data noted this as one of Riker's various battle techniques while preparing for another simulation, called Operation Lovely Angel, the first Starfleet battle simulation to be conducted following contact with the Borg. (TNG: "Peak Performance")
In 2370, alpha shift made up of Lieutenant Commander Data, Lieutenant Worf, and Ensigns Sito Jaxa and Sam Lavelle performed a battle drill on the bridge of the USS Enterprise-D. Sito was required to lock phasers onto a target while Lavelle directed the ship hard to starboard. Due to a change in course, Sito had to relock phasers before firing and the target was successfully destroyed. Commander Riker then ended the simulation sequence and reported that alpha shift's response time was seven percent slower than the gamma shift. (TNG: "Lower Decks")
A Starfleet ensign, who was stationed on Ajilon Prime in 2373, recalled his Academy days, where he "did really well in the battle simulations. I never had any problems," but in contrast to the Battle of Ajilon Prime, which was underway, "when you're out there and the live shells are detonating all around you, it's a whole different thing." Jake Sisko understood, first hand what the ensign meant, however, before this experience, he confided to Doctor Julian Bashir that he couldn't understand how "the people in [the ensign's] squad got scared and ran," because, "they're Starfleet. They've passed psych tests. They've spent hundreds of hours in battle simulations." Bashir sympathized, explaining, "[s]imulations can't prepare you for the real thing. Nothing can." (DS9: "Nor the Battle to the Strong")
Later that year, the injured Klingon General Martok used the excuse of turning off the holosuite safety protocols during a battle simulation for acquiring the shoulder injury Dr. Bashir had to repair. Martok, further adding that "spending two years in a Dominion prison camp has dulled my reflexes," and "[t]his only proves that I need further training." Martok later confessed his cover to Lieutenant Commander Worf, stating that Bashir "would not understand the need to draw blood in a warrior's training exercise." (DS9: "Soldiers of the Empire")
The following year, after Alexander Rozhenko joined the crew of the IKS Rotarran, he inadvertently forgot to erase the battle simulation program from the sensor display, causing the crew to react to a false attack from the Jem'Hadar. (DS9: "Sons and Daughters")