Who was the first king of the northern kingdom?
Jeroboam was the first king of the northern kingdom, which consisted of ten tribes. He seceded and later led the northern kingdom against Abijah of the southern kingdom of Judah and Benjamin, fielding 800,000 troops compared to Abijah's 400,000. This civil conflict is described in 2 Chronicles chapter 13.
Les explains that Jeroboam was the first king of the northern kingdom and the one who seceded. At this point both segments—the northern kingdom and the southern kingdom—are still in place and about ready for civil war. Abijah, from the southern kingdom of Judah and Benjamin, set an army of 400,000 chosen men in array. Jeroboam set his forces against Abijah with 800,000 troops, which is logical because ten tribes were in the north and only two in the south. Later in history, the northern kingdom could field only 580,000 while the southern kingdom had 1,160,000 because of a mass migration of Israelites from the northern kingdom down into the southern.
And Abijah set the battle in array with an army of valiant men of war, even four hundred thousand chosen men: Jeroboam also set the battle in array against him with eight hundred thousand chosen men, being mighty men of valour.