Final answer:
The installation of Maximilian as the ruler of Mexico was a test of the Monroe Doctrine because it contradicted the principles outlined in the doctrine.
Step-by-step explanation:
The installation of Maximilian as the ruler of Mexico was a test of the Monroe Doctrine because it directly contradicted the principles outlined in the doctrine. The Monroe Doctrine, issued in 1823, warned European nations to refrain from interfering with independent countries in the Western Hemisphere. By installing Maximilian, a European ruler, in Mexico, France was defying the Monroe Doctrine and asserting its influence in the Americas.