Complex sentence: Spain for a period had rule of New Orleans and all land west of the Mississippi, administering numerous French colonial towns, such as St. Louis and Ste. Genevieve in present-day Missouri.
Split into simpler sentences: Spain for a period had rule of New Orleans and all of the former French lands on the west side of the Mississippi River. They administered numerous historically-French colonial towns, such as St. Louis and Ste. Genevieve in present-day Missouri.