This invasion, however, also proved abortive, owing chiefly to the determined opposition displayed by the slave-holders both in the United States and Cuba. The ringleaders were captured and severely punished by the Spanish authorities. The struggles for freedom had attracted the attention of the people of the United States and were viewed by them with ever-increasing interest and sympathy. After the acquisition of Florida, the future of the island of Cuba became of more or less importance to the people of the United States and has remained so to the present day. As President Cleveland said in his message of December, 1896: "It is so near to us as to be hardly separated from our own territory." The truth of this is apparent when it is remembered that the straits of Florida can be crossed by steamer in five hours. It began to be feared that Cuba might fall into the hands of England or France and the governments of those countries as well as that of Spain were informed that such a disposition of it would never be consented to. Its position at the entrance of the gulf of Mexico could not be disregarded. The American government declared its willingness that it should remain a Spanish colony, but stated it would never permit it to become the colony of another country. In 1825 Spain made a proposition that, in consideration of certain commercial concessions the United States should guarantee to her the possession of Cuba; but this proposition was declined on the ground that such a thing would be contrary to the established policy of the United States. One of the most important consequences of Spain's efforts to regain possession of the South American republics, the independence of which had been recognized by the United States, was the formulation of what has since been known as the "Monroe Doctrine." In his message of December 2, 1823, President Monroe promulgated the policy of neither entangling ourselves in the broils of Europe, nor suffering the powers of the old world to interfere with the affairs of the new. He further declared that any attempt on the part of the European powers "to extend their system to any portion of this hemisphere" would be regarded by the United States as "dangerous to our peace and safety," and would accordingly be opposed. Although since then there has been more or less friction with England over the Monroe doctrine, at that time she greatly aided in its becoming established as a feature of international law, and strengthened the position of the United States, by her recognition of the South American republics.