Carl Friedrich Keil and Franz Delitzsch Old Testament Commentary

Judah invited Simeon his brother, i.e., their brother tribe, to take part in the contest. The epithet is applied to Simeon, not because Simeon and Judah, the sons of Jacob, were the children of the same mother, Leah (Gen 29:33, Gen 29:35), but because Simeon's inheritance was within the territory of Judah (Jos 19:1.), so that Simeon was more closely connected with Judah than any of the other tribes. "Come up with me into my lot (into the inheritance that has fallen to me by lot), that we may fight against the Canaanites, and I likewise will go with thee into thy lot. So Simeon went with him," i.e., joined with Judah in making war upon the Canaanites. This request shows that Judah's principal intention was to make war upon and exterminate the Canaanites who remained in his own and Simeon's inheritance. The different expressions employed, come up and go, are to be explained from the simple fact that the whole of Simeon's territory was in the shephelah and Negeb, whereas Judah had received the heart of his possessions upon the mountains.