 Emperor Nikephoros I invaded Bulgaria and sacked the capital of Pliska in 811, but was killed and his army annihilated at the Battle of Varbitsa Pass as he returned to Roman territory.Khan Krum's peace offer was rejected by Emperor Michael I Rangabe, and the Bulgarians invaded the Roman Empire in the spring of 812.In May, Krum besieged Debeltos and used siege engines to bombard the city.