Complex sentence: Part of the Walloon Guards were stationed in Madrid at the time of the French occupation in October 1808, and they were incorporated into the French Army, which already included a significant number of Belgian and Dutch units.
Split into simpler sentences: Part of the Walloon Guards were stationed in Madrid at the time of the French occupation in October 1808. These were incorporated into the French Army, which already included a significant number of Belgian and Dutch units.