Then all the Midianites and the Amalekites and the children of the east assembled themselves together; and they passed over, and encamped in the valley of Jezreel.
It is not known why only a few of the tribes of Israel were included in this call to arms, but it was probably some kind of a mistake due to the inexperience of Gideon.  The omission of Ephraim would later on confront Gideon with a problem.