Rehoboam is a son of Solomon, and he has inherited some of the good and some of the bad ideas his father left.  He had an army, and he was ready to fight against the many tribes that left to become Israel.  He got good counsel from a man of God.

And when Rehoboam was come to Jerusalem, he gathered of the house of Judah and Benjamin an hundred and fourscore thousand chosen men, which were warriors, to fight against Israel, that he might bring the kingdom again to Rehoboam.  But the word of the LORD came to Shemaiah the man of God, saying,  Speak unto Rehoboam the son of Solomon, king of Judah, and to all Israel in Judah and Benjamin, saying,  Thus saith the LORD, Ye shall not go up, nor fight against your brethren: return every man to his house: for this thing is done of me. And they obeyed the words of the LORD, and returned from going against  Jeroboam.  2 Chronicles 11:1-4

The man of God had the respect of Rehoboam, and the king decided to obey the word of God for now.  Thus civil war and shedding of Jewish blood was avoided.  But Rehoboam took no chances, building his fortifications against the larger tribes to the north.

And Rehoboam dwelt in Jerusalem, and built cities for defence in Judah.  He built even Bethlehem, and Etam, and Tekoa,  And Bethzur, and Shoco, and Adullam,  And Gath, and Mareshah, and Ziph,  And Adoraim, and Lachish, and Azekah,  And Zorah, and Aijalon, and Hebron, which are in Judah and in Benjamin fenced cities.  2 Chronicles 11:5-10

He went on to prepare the cities with food and weapons against the threat as well.  The men were organized into military units with a proper chain of command.

And he fortified the strong holds, and put captains in them, and store of victual, and of oil and wine.  And in every several city he put shields and spears, and made them exceeding strong, having Judah and Benjamin on his side.  And the priests and the Levites that were in all Israel resorted to him out of all their coasts.  For the Levites left their suburbs and their possession, and came to Judah and Jerusalem: for Jeroboam and his sons had cast them off from executing the priest’s office unto the LORD:  2 Chronicles 11:11-14

The holy men of Israel were coming to Jerusalem to serve God in the temple per the Lord’s commandment.  They were rejected from serving as priests by Jeroboam, so they were happy to join Judah. 

And he ordained him priests for the high places, and for the devils, and for the calves which he had made.  And after them out of all the tribes of Israel such as set their hearts to seek the LORD God of Israel came to Jerusalem, to sacrifice unto the LORD God of their fathers.  So they strengthened the kingdom of Judah, and made Rehoboam the son of Solomon strong, three years: for three years they walked in the way of David and Solomon.  2 Chronicles 11:15-17

Jeroboam made the priests serve idols and demons, but some remained true to the Lord and went south to Judah.

And Rehoboam took him Mahalath the daughter of Jerimoth the son of David to wife, and Abihail the daughter of Eliab the son of Jesse;  Which bare him children; Jeush, and Shamariah, and Zaham.  And after her he took Maachah the daughter of Absalom; which bare him Abijah, and Attai, and Ziza, and Shelomith.  And Rehoboam loved Maachah the daughter of Absalom above all his wives and his concubines: (for he took eighteen wives, and threescore concubines; and begat twenty and eight sons, and threescore daughters.)  2 Chronicles 11:18-21

Rehoboam began to collect wives as Solomon had.  Among them was the daughter of Absalom, who he loved.  So many partners must have had influence, and the rebellion of Absalom may have dwelt among the spirits being represented here. 

And Rehoboam made Abijah the son of Maachah the chief, to be ruler among his brethren: for he thought to make him king.  And he dealt wisely, and dispersed of all his children throughout all the countries of Judah and Benjamin, unto every fenced city: and he gave them victual in abundance. And he desired many wives.  2 Chronicles 11:18-21

The king did some things wisely after the world, but he desired many wives after his father Solomon.  He will be responsible and accountable for keeping the throne of David and Solomon against the works of darkness.