Complex sentence: Khan Krum engaged in an aggressive policy within the Balkans, raiding along the Struma valley in 807, where he defeated a Byzantine army and captured an enormous amount of gold intended as wages for the whole Byzantine army.
Split into simpler sentences: Khan Krum engaged in an aggressive policy within the Balkans, raiding along the Struma valley in 807. There he defeated a Byzantine army and captured an enormous amount of gold intended for wages of the whole Byzantine army.