Complex sentence: Many Spanish soldiers were killed by French and allied cavalry, while many others were made prisoner after their flight was cut short by the French 1st Corps that had not taken part in the battle, but had just crossed the Tagus.
Split into simpler sentences: Many Spanish soldiers were killed by French and allied cavalry. Many others were taken prisoner after their flight was cut short by the French 1st Corps, which had not taken part in the battle, but had just crossed the Tagus.