Complex sentence: Shots were fired by both sides, contemporary accounts indicate the Chartists attacked first but the soldiers holding the building despite being outnumbered by a large and very angry crowd had vastly superior fire-power, training and discipline all of which soon broke the crowd.
Split into simpler sentences: Shots were fired by both sides, contemporary accounts indicating that the Chartists attacked first. But the soldiers holding the building, despite being greatly outnumbered by the large and very angry crowd, had vastly superior fire-power, training and discipline, all of which soon broke the crowd.