As I understood it, the first Imperial troops to actually enter the Rebel Base on Hoth were Darth Vader and his landing party. The Imperial walkers were, at the time, still mixing it up with the Rebel ground forces. Darth Vader could not land on Hoth until the shield generator that protected the base from aireal assaults was knocked out. However, in the control room we hear "Imperial troops have entered the base!" about a minute before General Veers shoots and destroys the shield generator. Imperial troops could not have entered the base because the generator was still operational, hence, the base was still protected. We hear the same announcment again about a minute after the shield generator is destroyed, just before Darth Vader and the stormtrooper enter the control room.