Alma 49:17

And now behold, it came to pass, that when the Lamanites had found that Lehi commanded the city, they were again disappointed, for they feared Lehi exceedingly; nevertheless, their Chief Captains had sworn with an oath, to attack the city; therefore they brought up their armies.

When the Lamanites found that Lehi was in charge of the city, they were disappointed again. They were very scared of Lehi. But their captains had promised to attack the city. So they brought up their armies.

When the Lamanites realized Lehi was defending the city, their hearts sank. They were terrified of him. But their commanders had sworn an oath to attack, so they brought up their armies.