In Fallen Angels by Walter Dean Myers, during the first patrol in Tam Ky, what mistake does Lieutenant Gearhart make?

In Chapter 13, the squad sets up on a narrow path and right away, Sergeant Simpson is nervous. He feels the location is unsafe for the squad, but Lieutenant Gearhart says he knows what he is doing and to get the men set up. Richie is in charge of setting up a claymore weapon that fires metal shards for sixty meters. After sitting in position for a while, the squad begins firing. In the light of a flare, Richie sees a Vietcong turn his claymore around and the squad hits the ground, trying to avoid the metal shards. Richie is surprised he is not hit. Based on the number of muzzle blasts, Richie realizes his squad is outnumbered and his stomach turns. The muzzle blasts start getting farther away and Richie’s squad starts to retreat through a rice paddy. It starts to rain heavily and the squad waits a long time for the chopper to pick them up. Gearhart sets off a flare to alert the chopper and firing starts behind the squad. The new guy, Nate Turner, is hit and killed. Gearhart is visibly upset it is his fault the enemy found their location.