This verse is part of a pact made between Joab and his brother Abishai, commanders in King David's army, as they prepared for battle against a coalition of enemies: the Syrians and the Ammonites. It's a strategic agreement based on mutual support and covers the possibility of being outnumbered or facing a particularly strong foe.

Here's a breakdown of the meaning:

"He said, “If the Syrians are too strong for me, then you are to help me...": Joab, commanding one division of the army, is telling his brother Abishai, who commands another division, that if his forces are struggling against the Syrian army, Abishai should come to his aid. This recognizes that the Syrians might be a tougher opponent.

"...but if the children of Ammon are too strong for you, then I will help you.": Conversely, if Abishai's forces are overwhelmed by the Ammonites, Joab will shift his forces to assist Abishai.

In essence, the verse conveys:

A battle plan of coordinated support: They are agreeing to support each other based on where the greatest need arises on the battlefield.
Acknowledgement of varying enemy strength: They recognize the potential for one enemy (the Syrians) to be stronger than the other (the Ammonites), necessitating flexibility in their strategy.
Brotherly trust and alliance: The agreement demonstrates their commitment to working together and relying on each other for success, even if it means diverting resources from their own immediate objective.
Pragmatism: It's a realistic assessment of the situation and a contingency plan for dealing with potential setbacks. It prioritizes the overall success of the army over individual glory.

In short, it's a strategic agreement that maximizes their chances of victory by ensuring that neither division is left to be decisively defeated if the enemy has a local advantage. This is a key element of good military leadership and illustrates the importance of cooperation and mutual support in warfare.