Complex sentence: The bishop, observing this, thought the army was trying to return to Strasbourg, and thinking to crush them with his cavalry before they escaped, led them in a rapid advance, leaving his infantry to catch up as best they could.
Split into simpler sentences: The bishop, observing this, thought the army was trying to return to Strasbourg. Thinking to crush them with his cavalry before they escaped, he led this horsemen in a rapid advance, leaving his infantry to catch up as best they could.