Alma 59:9

And now as Moroni had supposed that there should be men sent to the city of Nephihah, to the assistance of the people to maintain that city, and knowing that it was easier to keep the city from falling into the hands of the Lamanites, than to retake it from them, he supposed that they would easily maintain that city;

Moroni had thought that soldiers would be sent to Nephihah to help protect the city. He knew it was easier to keep a city safe than to get it back after losing it. So he thought they would be able to keep that city safe.

Moroni had assumed the government would send reinforcements to Nephihah to help defend the city. He knew it was far easier to keep a city from falling than to retake it once lost, so he figured they’d hold it without trouble.