Second History was a Romulan project to change the shape of history and ensure that the United Federation of Planets never existed, thus making the Romulan Star Empire more powerful.
Initially, the Romulans were able to change history, and create an alternate timeline. However the results were not what the Romulans intended. While the United Federation of Planets did not exist in the timeline, the Vulcans were still powerful, and able to keep the Romulans from conquering all of known space. The Vulcans also helped the survivors of Earth, which had been devastated in a war brought about by Romulan tampering, to get back on their feet.
In this timeline, Spock was the commanding officer of the VSS ShiKahr, which was this timeline's version of the USS Enterprise. A number of people who served on board the Enterprise of the original timeline also served on the ShiKahr, including Doctor Leonard McCoy and Montgomery Scott. James T. Kirk served on the ship as an Ensign, whose career was sidelined after he was convicted of murder while in the Academy. Additionally, Christopher Pike had served as a first officer under Spock on the ShiKahr until he was given his own command.
Second History had also disrupted the fabric of space when it was implemented. By the mid 23rd century, the disruption had become severe enough that the universe began to collapse in upon itself. One of the effects of this disruption was that a number of people - including some highly ranked officers - had started going insane in 2269.
Ultimately, Second History was thwarted by Spock and Kirk, who traveled back in time to the late 22nd century to pinpoint when exactly the Romulans had interfered in the timeline. They prevented the Romulans from interfering, and the timeline created by Second History never occurred. (TOS novel: Killing Time)