The Senate came into its own as a foreign policy force between 1850 and 1870. For most of early American history, the House of Representatives was the dominant actor on international matters. Talented politicians, like Henry Clay and Albert Gallatin, bolstered the power of the lower chamber. That the Senate conducted its debates in secret until the early 1800s decreased its public profile. And most contentious issues regarding both domestic and foreign policy—such as the War of 1812 and the Missouri Compromise—originated in the House.
In the 1830s the Senate began its golden age, peopled by the "great triumvirate" of Clay, Daniel Webster, and John Calhoun. But it was not until the end of the Mexican War that foreign policy power shifted to the Senate. The rise of the Republican Party, the institutional effects of the slavery issue, and the fact that most key initiatives in 1850s foreign policy involved powers assigned to the Senate but not the House (such as treatymaking and confirming ambassadors) facilitated the transformation.
The final factor in this process came during and after the Civil War, when Massachusetts Senator Charles Sumner assumed the chair of the Senate Foreign Relations Committee. Sumner first attracted national attention during the Mexican War, when he delivered a public speech in Boston denouncing the conflict as immoral. He became a household name after being caned—in the Senate chamber—by proslavery Representative Preston Brooks.
As Foreign Relations Committee chair, Sumner demonstrated his political skills, showing how he could use the institutional powers of the Senate to rally support even from colleagues that did not necessarily share his approach to international affairs. Sumner most made his influence felt in 1870, when he almost single-handedly blocked President Ulysses S. Grant's treaty to annex the Dominican Republic. Future Foreign Relations Committee chairs of both parties—figures such as Augustus Bacon, Henry Cabot Lodge, William Borah, Arthur Vandenberg, and J. William Fulbright—built on Sumner's precedents.