Excerpts from Books and Wikipedia
In 1550 BC, Egypt was reunified under Ahmose at Thebes to begin a prosperous period; [Grammar Ancient World, p. 15]
"It had taken Ahmose twenty years to take back Lower Egypt, and both Manetho and Josephus give him a rule of twenty-five years. He did not enjoy his position as king of all Egypt for long. But for his reunification of Egypt, and his reassertion of native Egyptian rule over the kingdom, Manetho names him the first king of the Eighteenth Dynasty. With this reunification, Egypt enters into a new phase of building, of peace and prosperity, of art and literature: the New Kingdom." [Bauer: Ancient World, p. 205]