Here the front of the army is spoken of as if it were the head. Abijah is saying that God is the one who is leading the army of Judah in battle. Alternate translation: "God is with us as our leader" (See: [[rc://en/ta/man/jit/figs-metaphor]])
Since Yahweh is leading the army of Judah, Abijah speaks of the army of Israel fighting against the army of Judah as if they were fighting against Yahweh himself. (See: [[rc://en/ta/man/jit/figs-explicit]])