Final answer:
The Mississippi River was the natural feature that divided Spanish Territory from British Territory after the French and Indian War, as per the Treaty of Paris of 1763.
Step-by-step explanation:
The natural feature that divided Spanish Territory from British Territory after the French and Indian war was the Mississippi River. The Treaty of Paris of 1763 ended the French and Indian War with Great Britain taking control of territories east of the Mississippi River, except for New Orleans and Louisiana. While these lands were transferred to Britain by France, Spain, having sided with France during the war, ended up with Louisiana and had to cede Florida to the British. Nonetheless, after the war, the British and the Spanish engaged in disputes over the exact boundaries, with Spain claiming the Tennessee River as the border between the United States and New Spain. The Mississippi River thus became a critical geographical and political feature separating British and Spanish claims.