The essential verse here is verse 20 where Gideon delivers the message to Limhi that the armies of the Lamanites are gathering once again to come up against the Limhites. While the Limhites had one a decisive battle, the issue of the breaking of the oath was still current with the Lamanites. The loss of their king would have occasioned the selection of a new leader, and the reason for the original invasion remained. Thus it is not surprising that the Lamanites were preparing for another assault on Lehi-Nephi.
In this situation Gideon sees the fulfillment of prophecy, but not a fatal one. They are being afflicted, but now need to save their land. What Gideon proposes is that the reason for the war be eliminated. The king of the Lamanites will plead for them because of honor - the very thing that led to the war in the first place. The conflict is justified because of the breach of an oath. If Limhi's people did not break their oath, then the reason for the war can be removed, and the conflict aborted.