Complex sentence: The final force was Lieutenant Perriman's infantry platoon, who were tasked with securing a bridge in the centre of An Nasariyah, along the route the convoy would take.
Split into simpler sentences: The final force was a securing force, which consisted of Lieutenant Perriman's infantry platoon. They were tasked with securing a bridge in the centre of An Nasariyah, along the route the convoy would take.