Why did Yo'av blow the Shofar?
Radak: They had gathered, and he could not stop them [any other way].
What is the meaning of "v'Lo Yirdefu"?
Radak: They did not chase. It is past tense written in the future.
Kli Yakar: Yo'av and his men agreed not to pursue Yisrael any more. Indeed, they did not fight them after this; Avner was killed, and they did not fight more 1 .
Malbim: They did not chase the scattered men who fled [and they did not fight the men who gathered].
"The war stretched out between Beis Sha'ul and Beis David" (3:1)! Perhaps Kli Yakar means that David's men did not pursue Avner's men; the latter initiated all conflicts. This requires investigation. (PF)