Complex sentence: The Seventh Army drove forward on the northern flank and advanced elements reached Breda on 11 May, by when the Germans had captured the north-eastern frontier provinces of the Netherlands, were advancing on The Hague ("Den Haag") and fighting in Rotterdam.
Split into simpler sentences: The Seventh Army drove forward on the northern flank and advanced elements reached Breda on 11 May. The Germans had captured the north-eastern frontier provinces of the Netherlands, were advancing on Den Haag and fighting in Rotterdam.