Sandusky native Jay Cooke didn't win the Civil War by himself, but he did contribute to the Union's victory by helping to finance the North's war effort, according to "The Battle Cry of Freedom," the Pulitzer Prize-winning history of war by James McPherson.
Chapter 14 of the book, which I recently began reading, explains that President Lincoln's secretary of the treasury, Salmon P. Chase, knew little about banking but was a quick learner. Jay Cooke was Chase's "principal tutor," the book says.
"Chase pioneered the concept of selling bonds to ordinary people, as well as to bankers, in denominations as small as $50 to be paid in monthly installments. Cooke undertook to market these bonds by patriotic advertising that anticipated the great war-bond drives of the 20th century," the book says.
If you want to learn more about Cooke, Ellis P. Oberholtzer's two volume 1907 biography, "Jay Cooke: Financier of the Civil War" has entered the public domain and is a free download.