On April 14th, 1865 Lewis Powell stabbed Secretary of State William Henry Seward to death at Seward's home in Washington D.C. This was part of John Wilkes Booth's conspiracy to assassinate the heads of the United States's Executive Branch. Powell was captured at Mary Surratt's boarding house and executed, by hanging, on July 7th, 1865.
Seward was an advocate of American expansion throughout the Western Hemisphere. He was also an advocate of the Monroe Doctrine, which aimed to prevent European expansion in the Americas. After Seward's death, President Andrew Johnson's administration was consumed by the post-Civil War Reconstruction of the South and largely ignored foreign affairs.
Seward was an outspoken critic of the Second French Empire's occupation of Mexico in 1861 and the installation of Austrian archduke Maximilian as Emperor of Mexico in 1864. Seward was unable to intervene, however, because of the ongoing American Civil War. After the war, the Johnson administration was too occupied with the Reconstruction of the South, and later with Johnson's impeachment, to directly deal with Mexico, although General Ulysses S. Grant and General Philip Sheridan covertly aided the Mexican resistance.
In 1867 Eduard de Stoeckl, the Russian envoy to the United States, approached the US State Department. Russia wanted to sell it's North American land to the United States, but the Johnson Administration saw little value the largely undeveloped, sub-arctic Russian America, and passed-up the offer.