The Long Winter is more of an event than a war, but it was at this time period that many battles were fought and many lives were lost. Rohan and Eriador suffered the most from this event, and the enemies of both Gondor and Rohan took advantage of the harsh weather. This was an eerie and dark time for men and hobbits. But the Free Peoples held fast and clung to hope--despite losing thousands of lives, Gandalf grew fond of the Shire folk at this time.
In Rohan, Helm held and fought against the enemies of Rohan and Gondor, though a subject of conjecture, it has been often thought and explained that Bladorthin of Dorwinion/Rhovanion may have had some hand in the battles during T.A. 2758 and thus indirectly caused the numbers of Easterlings to be reduced when they assaulted Rohan.
Battles during the Long Winter War: