After the opening of the island to world trade in 1818, trade agreements began to replace Spanish commercial connections. In 1820 Thomas Jefferson thought Cuba is "the most interesting addition which could ever be made to our system of States" and told Secretary of War John C. Calhoun that the United States "ought, at the first possible opportunity, to take Cuba." In a letter to the U.S. Minister to Spain Hugh Nelson, Secretary of State John Quincy Adams described the likelihood of U.S. "annexation of Cuba" within half a century despite obstacles: "But there are laws of political as well as of physical gravitation; and if an apple severed by the tempest from its native tree cannot choose but fall to the ground, Cuba, forcibly disjoined from its own unnatural connection with Spain, and incapable of self support, can gravitate only towards the North American Union, which by the same law of nature cannot cast her off from its bosom."