The most serious strain to which the Monroe Doctrine was ever subjected was the attempt of Louis Napoleon during the American Civil War to establish the empire of Maximilian in Mexico under French auspices. He was clever enough to induce England and Spain to go in with him in 1861 for the avowed purpose of collecting the claims of their subjects against the government of Mexico. Before the joint intervention had gone very far, however, these two powers became convinced that Napoleon had ulterior designs and withdrew their forces. Napoleon's Mexican venture was deliberately calculated on the success of the Southern Confederacy. Hence, his friendly relations with the Confederate commissioners and the talk of an alliance between the Confederacy and Maximilian backed by the power of France. Against each successive step taken by France in Mexico Mr. Seward, Lincoln's Secretary of State, protested. As the Civil War drew to a successful conclusion his protests became more and more emphatic. Finally, in the spring of 1866, the United States Government began massing troops on the Mexican border and Mr. Seward sent what was practically an ultimatum to the French Emperor; he requested to know when the long-promised withdrawal of the French troops would take place. Napoleon replied, fixing the dates for their withdrawal in three separate detachments. American historians have usually attributed Napoleon's backdown to Seward's diplomacy supported by the military power of the United States, which was, of course, greater then than at any previous time in our history. All this undoubtedly had its effect on Napoleon's mind, but it appears that conditions in Europe just at that particular moment had an even greater influence in causing him to abandon his Mexican scheme. Within a few days of the receipt of Seward's ultimatum Napoleon was informed of Bismarck's determination to force a war with Austria over the Schleswig-Holstein controversy. Napoleon realized that the territorial aggrandizement of Prussia, without any corresponding gains by France, would be a serious blow to his prestige and in fact endanger his throne. He at once entered upon a long and hazardous diplomatic game in which Bismarck outplayed him and eventually forced him into war. In order to have a free hand to meet the European situation he decided to yield to the American demands. As the European situation developed he hastened the final withdrawal of his troops and left Maximilian to his fate. Thus the Monroe Doctrine was vindicated!