Clan War. Highland Heroes, #1

An evil wizard is using experimental technology to travel through time to assassinate the President of the United States. Only one group of US Army veterans can stop the disaster from consuming all of human history, but they'll have to face untold dangers and mysterious forces to do it. After a disastrous mission wipes out their division in Afghanistan, the five surviving members retire to the Detroit slums hoping to dedicate their lives to serving humanity.
Lily Dindle is living peacefully at the Last Division's headquarters when her brother Liam breaks into their armed compound spinning a wild tale of time-travel, deadly wizards, and the plot to assassinate the President. Liam takes Lily through the time portal alone to ancient Scotland, first to search for Lady Rhona Armstrong, the President's ancestor the wizard has travelled back in time to kill.
On the other side of the portal, Grant and Elliot Ritchie are risking their lives to protect Prince Ness Creighton from a murder plot by Clan Buchanan, the mortal enemies of Clan Creighton. Grant doesn't know what to think when Lily tells him about the plot against Lady Rhona even though no such person has ever existed. Lily quickly realizes that this isn't the Scotland she thought it was, but an alternate reality populated by dragons, otherworldly shifters, and dangers she never thought possible.
The party teams up, only for Laird Balfour Creighton to turn against them. All four must fight and run from their lives, not just from the deadly Buchanans but from the Laird's forces that threaten to wipe the travelers out at every turn.