Complex sentence: By now, Charles was acutely aware that his troops would not hold out much longer: his three Korps on the Wagram plateau (I, II and IV) had been in action for some ten hours.
Split into simpler sentences: By now, Charles was acutely aware that his troops would not hold out much longer. His three corps on the Wagram plateau (I, II and IV) had been in action for some ten hours.