Asa succeeded Abijah as king of Judah.  His reign was long and righteous, at least as compared with that of his rivals in Israel.  But it was not peaceful:

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         There was war between Asa and Baasha king of Israel throughout their reigns (1 Kings 15:16).

        

But the Book of 2 Chronicles (only) reports an even more serious threat during Asa's reign:

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         Zerah the Cushite marched out against them with a vast army and three hundred chariots, and came as far as Mareshah (2 Chron 14:9).
    
        But who was Zerah?

       

The probability is that this was an Egyptian followup campaign to Pharaoh Shisak's successful and profitable looting of Jerusalem during the reign of Rehoboam.  Zerah may not have been the name of the Pharaoh, but that of the prince or general in charge (see NIV, p. 639 footnote).  The army itself was Egyptian, and its leader was Cushite, probably an Ethiopian.   However, the Egyptians did not repeat their previous success.  Asa cried out to the Lord for help, and the armies of Judah and Benjamini defeated the army of a thousand thousands, pursued them and raided cities.  

       

This seemed to have sparked a mini-revival in Judah.  God spoke to Asa a word of encouragement through a prophet named Azariah, and he responded by repairing the altar, gathering the people (including the refugees from Israel), offering sacrifices to God and entering into a covenant of obedience (2 Chron 15).
       
The good times did not last long.  Things got so bad between Israel and Judah that not only did they fight one another, but they even made treaties with non-Israelites against each other.  King Asa took the plunder from the defeat of Zerah and sent it to Ben-Hadad of Aram to bribe him to break his alliance with Israel (1 Kings 15:15-21).  Ben-Hadad then began to attack Israel, which ended King Baasha's advance on Judah.  God was displeased with Asa's action and rebuked him through the prophet Hanani.  He reminded Asa that it was due to His help that the Egyptians had been defeated, so why had he not trusted God when Israel attacked him? (2 Chron 16:7-10).  Asa's response -- he put the prophet in prison!