Complex sentence: The corps, containing the 9th SS Panzer and 10th SS Panzer Divisions refitting after Normandy, had been overlooked by Allied intelligence: while still seriously understrength, it was composed of veteran troops and was a deadly threat to lightly equipped paratroopers.
Split into simpler sentences: The corps, containing the 9th SS Panzer and 10th SS Panzer Divisions refitting after Normandy, had been overlooked by Allied intelligence. Whilst still seriously understrength, it was composed of veterans and composed was a deadly threat to lightly equipped paratroopers.