Dearborn fought at the Battle of Monmouth in New Jersey in 1778, following the British evacuation of Philadelphia to retreat to concentrate at New York City, in the final major battle of the Northern Theatre, and in the summer of 1779 he accompanied Major General John Sullivan on the Sullivan Expedition against the Iroquois in upstate New York and in the Battle of Wyoming against the Six Nations, thereafter laying waste to the Genesee Valley and the various regions around the Finger Lakes.