On the eve of the Civil War and after, Illinois was one of the most significant states in the Union. Its history is, in many respects, the history of the Union writ large: its political leaders figured centrally in the war's origins, progress, and legacies; and its diverse residents made sacrifices and contributions--both on the battlefield and on the home front--that proved essential to Union victory.

The documents in Illinois's War reveal how the state and its people came to assume such a prominent role in this nation's greatest conflict. In these crucial decades...