Complex sentence: After Mexico's independence in 1821, a large number of Spanish troops continued to occupy San Juan de Ulúa as late as 1825, when they were finally expelled by President Vicente Guerrero after a failed attempt at re-conquering the country.
Split into simpler sentences: After Mexico's independence in 1821, a large number of Spanish troops continued to occupy San Juan de Ulúa as late as 1825. The Spanish forces were expelled by President Vicente Guerrero after the failed attempt at re-conquering the country.