Complex sentence: At the beginning of its history, Mitanni's major rival was Egypt under the Thutmosids, but with the ascent of the Hittite empire, Mitanni and Egypt made an alliance to protect their mutual interests from the threat of Hittite domination.
Split into simpler sentences: At the beginning of its history, Mitanni's major rival was Egypt under the Thutmosids. However, with the ascent of the Hittite empire, Mitanni and Egypt made an alliance to protect their mutual interests from the threat of Hittite domination.