In an alternate timeline in which Khan Noonien Singh won the Eugenics Wars and Humanity underwent genetic engineering en masse, the Breen War was a conflict fought between the Khanate of Earth and the Breen Confederacy during the 24th century.
During the war, the Breen staged numerous attacks in Khanate space which had cost the Khanate dearly. Among the casualties were libraries of contemporary history, meaning that the Children of Khan possessed only fragmentary knowledge of the history of the late 20th and 21st centuries. However, the survival of any records of Earth's ancient cultures was credited to Khan's interest in his ancestors.
At one point, the Khanate developed a countermeasure against the energy dampening weapon prior to the engagement against the Breen, and managed to overpowered them eventually.