Helaman 1:18

And it came to pass that because of so much contention and so much difficulty in the government, that they had not kept sufficient guards in the land of Zarahemla; for they had supposed that the Lamanites durst not come into the heart of their lands to attack that great city Zarahemla.

Because of all the fighting and problems in the government, they had not left enough guards in Zarahemla. They thought the Lamanites would not dare come into the middle of their lands. They thought they would not attack the great city of Zarahemla.

Because of all the internal fighting and political chaos, the Nephites hadn’t posted enough guards around Zarahemla. They never imagined the Lamanites would be bold enough to strike at the heart of their territory and attack their capital.