The Alaska Purchase was the United States' acquisition of Alaska from the Russian Empire. Alaska was formally transferred to the United States on October 18, 1867, through a treaty ratified by the United States Senate. Russia had established a presence in North America during the first half of the 18th century, but few Russians ever settled in Alaska. In the aftermath of the Crimean War, Russian Tsar Alexander II began exploring the possibility of selling Alaska, which would be difficult to defend in any future war from being conquered by Russia's arch rival, the United Kingdom. Following the end of the American Civil War, U.S. Secretary of State William Seward entered into negotiations with Russian minister Eduard de Stoeckl for the purchase of Alaska. Seward and Stoeckl agreed to a treaty on March 30, 1867, and the treaty was ratified by the United States Senate by a wide margin. The purchase added 586,412 square miles (1,518,800 km2) of new territory to the United States for the cost of $7.2 million 1867 dollars. In modern terms, the cost was equivalent to $133 million in 2020 dollars or $0.37 per acre.