William Henry Seward wanted the United States to purchase Alaska, which was offered at a bargain price by the cash-strapped Tsar of Russia. While he was roundly mocked and Alaska was called Seward's Folly, the rich natural resources and strategic placement of Alaska ultimately justified his advice.
In 1864, Secretary of State Seward learned that Alaska might be for sale by the Russians. He inquired about the possibility to Russian minister Eduard de Stoeckl, who urged the Tsar to agree to a sale for fear that American or British Canadian settlers might overrun the lightly populated territory anyway. In 1867, Stoeckl returned to Seward with an offer and the authority to negotiate on the Tsar's behalf, and the sale of Alaska went through for $7.2 million. In spite of the ridicule directed at Seward's accomplishment, he was vindicated first in 1896 when a large gold deposit was discovered in Alaska's Klondike region and again when Alaska proved to be strategically critical in World War II and the Cold War.
Seward, a strong proponent of American expansionism, also wanted the Danish East Indies, today's American Virgin Islands, as a location to put an American naval base, and he negotiated with the Dominican Republic for an American port. Neither of these happened during his lifetime, but in 1916 the Danish did sell their Caribbean territory to the United States.