Complex sentence: He took part in the Battle of Vimeiro, and the expedition into Spain under Sir John Moore, in which the Rifles bore the brunt of the rearguard fighting.
Split into simpler sentences: He took part in the Battle of Vimeiro, and the expedition into Spain under Sir John Moore. The Rifles were present at Vimeiro, and in the campaign of Sir John Moore they bore the brunt of the rearguard fighting.