We are not told precisely the amount of time that passed between the resolution of the previous war and this new increase in hostilities. The only indication is that it is "after many days." This could be a few days, a week or more, or perhaps a month. It would not appear to be a year, for then it would be easier to note that a year had passed (particularly since there are indications that the large plates record was kept by years).
This tells us that the increase in hostilities is close in time to the previous event, and therefore likely to be based upon the same reasons. Since the previous conflict regarded the oath itself, what is creating this conflict? Mormon does not tell us. We might speculate, however, that while there was an official recognition that the Limhites might not have broken the oath through the abduction of the daughters of the Lamanites, there was nevertheless no firm evidence that the priests of Noah really were the culprits. This would have created a situation where there was an official reinstatement of the treaty, but a personal hesitation on the part of individuals among the Lamanites. The official treaty might be in place, but they would likely suspect that the story of the priests of Noah was a tale of convenience; one that they did not believe. If they believed that the Limhites were indeed guilty, the resumption of hostilities is understandable.