Under General Kesselring, German forces were making very effective use of the many river valleys traversing the Allied lines of advance. Ultimately, their goal was not to halt the advance but rather to delay the Allies whilst a more formidable stop line – the Gustav Line - was constructed further north around Monte Cassino.
The defences in the Capuan Plain were made up of a series of strong-points based around machine-gun nests and mortar positions zeroed in on likely crossing points. They were able to observe movement from a number of farm ruins, getting ample warning of British intentions.
Additionally, unseasonal torrential rain had turned the Volturno from a trickle to a deep and flowing river.