Three Michigan officers played major parts in the Battle of Antietam : General Alpheus S. Williams, General Israel Richardson, and General Orlando Willcox.
In total, several thousand Michiganders fought; Michigan forces claimed approximately 360 casualties during the battle, the single bloodiest day in U.S. military history.
Whether one claims that Antietam was a draw or a victory, the battle enabled President Lincoln to issue the preliminary Emancipation shortly thereafter. And it prevented England and France from recognizing the Confederate States of America.
Sources:
Scott Pohl, “Authors tell story of Michigan’s role at Antietam“, “ WKAR News, November 23, 2015.
Tom Bowman, “Antietam: A Savage Day In American History”, NPR, Septembe 17, 2012.
“Michigan at Antietam Day Honors CW Soldiers’ Sacrifices”, Michigan History, January/Febrary 2013, p.6.
Roger L. Rosentreter, “Three Generals and an unlucky regiment: Michigan at the Battle of Antietam”, Michigan History Magazine. Sept-Oct, 2008, Vol. 92 Issue 5, p20, 10 p
Michigan at Antietam : the Wolverine State’s sacrifice on America’s bloodiest day / Jack Dempsey & Brian James Egen
Antietam by the Civil War Trust.