2 Samuel 10:11
And he said, If the Syrians be too strong for me, then thou shalt help me: but if the children of Ammon be too strong for thee, then I will come and help thee. And he said, If the Syrians be too strong for me, then thou shalt help me: but if the children of Ammon be too strong for thee, then I will come and help thee.
The agreement belongs to the war that followed the Ammonites' insult to David's ambassadors. The conflict began when David's peaceful message of sympathy was misread as hostile, and the Ammonites formed an alliance against Israel. Within that defensive struggle against the Ammonite alliance with the Syrians, the commanders of Israel agreed to help each other against whichever enemy proved too strong.
Instead of waiting for invasion by the Ammonite and Syrian alliance, Joab led Israel across the Jordan toward the Ammonite capital. He urged the army to be courageous and valiant for their people and for the cities of God, while leaving the outcome to the Lord to do what is good in His sight. This offensive faithfulness led to the united allied forces being overcome in the first battle. After renewed war the next year, the Syrians from Lebanon to the Euphrates gave up the war and became tributary to Israel, and the Ammonite strongholds fell under Israel's dominion.