Where were the scouts when the titans attacked in the beginning? Where were Levi and the scouts during the original attack in episode 1 when the titans first broke through the wall?
The scouting regiment was wiped out by the colossal titan before the attack on Wall Maria and those who were not eaten were too injured to fight, so they could not fight the titan invasion. During the fall of Trost, and the battle for Trost, the Levi squad and the Survey Corps were gone on an expedition outside of Wall Rose, finally returning just in time to save the remnants of the soldiers sent to help get Trost back from the titans.