The War of Gundabad was fought between the Orcs of Gundabad and the Reunited Kingom, Elves, and Dwarves of the Blue Mountains against Gundabad to repel the remnant orc forces of Sauron's armies and liberate northern Eriador. It took place in the beginning of the Fourth Age.
The war began when hordes of orcs descended from Angmar into northern Eriador, having lost the fortress in Gundabad to the combined efforts of the Dwarves and Elves. Given the scarcity of the armies between both the Dwarves and Elves, the mantle of spearheading the campaign came down to the recently reforged Kingdom of Arnor. With the morale high after the War of the Ring, most of the casualties were exclusively orcs. The war signified the restoration of the lands north of Fornost and the final destruction of Angmar and Gundabad.
The war marked the heralding of King Elessar's distant relative, Earorfilas, who led Arnor's emergent armies as their commander. This also allowed the Reunited Kingdom to reclaim the ancient borders of Arnor centuries before. The long last ancient blade Ringil was reclaimed by Eärorfilas during this time.