Complex sentence: They attempted to capture Baton Rouge in 1804, but were defeated, having failed to gain the support of local Anglo-American settlers, most of whom were satisfied with Spanish rule on account of Spain's liberal land grants and its protection of slavery.
Split into simpler sentences: They attempted to capture Baton Rouge in 1804, but were defeated, having failed to gain the support of local Anglo-American settlers. Most of the latter were satisfied with Spanish rule on account of Spain's liberal land grants and its protection of slavery.