John Gill Commentary 2 Samuel 10:15

"And when the Syrians saw that they were put to the worse before Israel, they gathered themselves together." — 2 Samuel 10:15 (ASV)

And when the Syrians saw that they were smitten before Israel Considered it in their minds, and conversed with one another about it, and fearing what would be the consequence of it:

they gathered themselves together; not only the forces got together again, that fled before Joab, but all the kings of Syria united their forces together, as appears from (2 Samuel 10:19); supposing that David would avenge himself on them for assisting the Ammonites against him; and therefore judged it advisable to raise a large army, that they might be in a condition to receive him.