The Second Romulan War was a war between the United Federation of Planets and the Romulan Star Empire sometime in the 25th century of an alternate timeline. It started because the Order of Omega used Omega molecules to destabilize space, making large portions of the Alpha Quadrant impassible to warp travel. The Romulans were implicated in the crime, leading to war with the Federation. According to Daniels, they were able to isolate this timeline. (DTI novel: Watching the Clock)