This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
(covers information from several alternate timelines)
In 2016, a Vulcan starship crossed into Klingon space near H'atoria and was immediately destroyed by the Klingons. In the aftermath, Vulcans established a policy of firing first when encountering Klingons, earning their respect and eventually leading to the establishment of formal relations. (DIS: "The Vulcan Hello", "Magic to Make the Sanest Man Go Mad")