The M-5 drills were, according to Commodore Bob Wesley, "a series of routine research and contact problems for the M-5 to solve, plus navigational maneuvers and the war games problem." After having been previously successful in performing simulated tests, Starfleet decided to test the device under actual conditions. The ultimate goal was said to realize a new "revolution in space technology as great as warp drive."
The Lexington leads the charge
In 2268, Starfleet conducted an elaborate simulation involving Doctor Richard Daystrom's experimental M-5 computer, designed to autonomously operate all starship functions without the input of a ship's crew. The USS Enterprise was equipped with the M-5 multitronic unit to test its ability to function in several areas. As a test of the M-5's tactical and strategic capabilities, four other Constitution-class starships were assembled to launch a coordinated attack against the Enterprise. Commodore Wesley, commanding the USS Lexington, led the attack force, which also included the USS Excalibur, the USS Hood, and the USS Potemkin.
However, the M-5 malfunctioned during the simulated attack; rather than firing with minimum-powered phasers, the M-5 attacked the battle group with full-powered phasers and photon torpedoes, causing substantial death and damage to the fleet as well as completely crippling the Excalibur and killing everyone on board. Enterprise Captain James T. Kirk managed to force the M-5 to deactivate before the remaining starships launched a final attack to destroy the Enterprise. (TOS: "The Ultimate Computer")