Jay Cooke was a pioneering American financier who revolutionized investment banking and played a crucial role in financing the Union's efforts during the Civil War. Through his firm, Jay Cooke & Company, he marketed war bonds directly to the public in small denominations, making it possible for ordinary citizens to invest in the war effort. After the war, he became the principal financier for the Northern Pacific Railway, but his firm's subsequent failure was a major precipitating factor in the Panic of 1873. Despite this major setback, Cooke later rebuilt his fortune and is remembered as a central figure in the development of modern American finance.