Albert Barnes Commentary 2 Samuel 10:12

"Be of good courage, and let us play the man for our people, and for the cities of our God: and Jehovah do that which seemeth him good." — 2 Samuel 10:12 (ASV)

For the cities of our God - This suggests that the relief of Medeba was one of the immediate objectives, and consequently, that Medeba was still in the possession of the Reubenites at this time. To prevent an Israelite city from falling into the hands of a pagan people, and the rites of Moloch from being substituted for the worship of Yahweh, was a very urgent motive for valor.