2 Chronicles 13:12
And, behold, God himself is with us for our captain, and his priests with sounding trumpets to cry alarm against you. O children of Israel, fight ye not against the LORD God of your fathers; for ye shall not prosper. And, behold, God himself is with us for our captain, and his priests with sounding trumpets to cry alarm against you. O children of Israel, fight ye not against the LORD God of your fathers; for ye shall not prosper.
The verse belongs to the account of war between Judah and Israel in the reigns of Rehoboam and Abijah, in which Judah conquered. In it Abijah warns Israel not to fight Judah because God himself is with Judah as its captain, with his priests sounding trumpets to cry alarm against Israel. That sounding of trumpets shows the regular worship and trumpet sounding continuing at the Temple as in the tabernacle service. The warning rests on God's covenant with David, by which the kingdom was given to David and his descendants by an enduring covenant of salt.
Judah's victory over Israel came through the Lord's help when Abijah called upon him, with the priests sounding their trumpets and the people shouting, and Israel was defeated in the battle that follows. To fight against the Lord God of their fathers is to undertake what will not prosper. God is with his people as their Captain, and their strength in conflict comes from the Holy Ghost as they contend by his power for truth, righteousness, holiness, love, and God's honor.
Applied to the American Civil War, the verse warns that a government opposing God by supporting slavery fights against God and cannot prosper. Had the North followed Abijah's example the rebellion would have ended.