Final answer:
France posed a threat to Spanish lands in 1800 as they were at war with other European nations and Spain was an ally of France. This led to the Peninsular War involving Spanish, Portuguese, and British forces fighting against the French.
Step-by-step explanation:
The country that posed a threat to Spanish lands in 1800 was France. During the Napoleonic Era, France was at war with other European nations, and Spain was an ally of France. As a result, Portugal, which was a long-standing ally of Great Britain, became a target for invasion by France. This conflict, known as the Peninsular War, involved Spanish, Portuguese, and British forces fighting against the French.