A Starfleet directive requiring that every attempt at first contact and a peaceful settlement be made before engaging an alien species in battle, cited over Species 8472.
Directive 010 states that before engaging an alien species in battle, any and all attempts to make first contact and reach a non-military resolution must be made. It is quoted in the Voyager episode In the Flesh.
Voyager discovers that Species 8472 has built a detailed copy of Starfleet Headquarters on a space station, staffed by aliens altered to pass as humans, in what looks like training for an invasion of Earth. Janeway's first instinct is to arm nanoprobe warheads, the weapon that defeated the species before.
The directive is raised as the argument for talking first, and the story follows that course. Janeway meets the aliens' leaders, who wear the forms of Boothby and a Starfleet admiral, and both sides step back from an attack that each feared the other was planning.