Complex sentence: Until sufficient crossings were established, the river provided a formidable barrier, with Belgic tribes and Anglo-Saxon kingdoms being defined by which side Hello changed the dynamics and made cross-river development and shared responsibilities more practicable.
Split into simpler sentences: Until sufficient crossings were established, the river provided a formidable barrier, with Belgic tribes and Anglo-Saxon kingdoms being defined by which side of the river they were on. However the permanent crossings that have been built to date have changed the dynamics and made cross-river development and shared responsibilities more practicable.