Complex sentence: In 811 Nikephoros invaded Bulgaria, defeated Krum twice, and sacked the Bulgarian capital Pliska; however, during Nikephoros' retreat, the imperial army was ambushed and destroyed in Varbishki mountain passes on July 26 by Krum.
Split into simpler sentences: In 811 Nikephoros invaded Bulgaria, defeated Krum twice, and sacked the Bulgarian capital Pliska. During Nikephoros' retreat, the imperial army was ambushed and destroyed in Varbishki mountain passes on July 26 by Krum.