National Archives and Records Administration. Office of the Federal Register. Record Group 11: General Records of the United States Government, 1778 - 2006. Enrolled Acts and Resolutions of Congress, 1789 - 2011. National Archives Identifier: 299950. Act of June 18, 1812, 2 STAT 755, Declaration of War with Great Britain, War of 1812. Available from http://research.archives.gov/description/299950 or http://media.nara.gov/rediscovery/08305_2007.pdf.
On 18 June 1812, the United States Congress passed an act declaring war on the United Kingdom of Great Britain and Ireland, and the War of 1812 began.
Some of my family members participated in the War of 1812. The ones that I have identified so far are my 6th-great-grandfather Francis Hardgrave and his son Skelton Hardgrave, my 5th-great-grandfather Andrew Russell (Francis Hardgrave's son-in-law), and my 5th-great-grandfather Chapman Gordon.