Here "Judah" represents the soldiers in the army of Judah. AT: "When the soldiers of Judah looked back" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The word "behold" here shows that the soldiers were surprised by what they saw.
# the fighting was both in front of them and behind them
The word "fighting" can be translated as a verb. AT: "the army of Israel was fighting them from the front and from the back" (See: [[rc://en/ta/man/translate/figs-abstractnouns]])
# God struck Jeroboam and all Israel before Abijah and Judah
God enabling the army of Judah to defeat Jeroboam and the army of Israel is spoken of as if God struck Jeroboam and Israel. AT: "God enabled Abijah and the army of Judah to defeat Jeroboam and all Israel" (See: [[rc://en/ta/man/translate/figs-metaphor]])