John Gill Commentary 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." — 2 Samuel 10:11 (ASV)

And he said, if the Syrians be too strong for me
Which he might perceive by Joab's forces giving way, or by some signal agreed on between them

then thou shall help me ;
detach a part of his army to his support and assistance:

but if the children of Ammon be too strong for thee, then I will come
and help thee ;
in the same manner.