William Rosecrans, American general, politician, and diplomat, United States Ambassador to Mexico (d. 1898)
William Starke Rosecrans (September 6, 1819 – March 11, 1898) was an American inventor, coal-oil company executive, diplomat, politician, and U.S. Army officer. He gained fame for his role as a Union general during the American Civil War. He was the victor at prominent Western Theater battles, but his military career was effectively ended following his disastrous defeat at the Battle of Chickamauga in 1863.

Rosecrans graduated in 1842 from the United States Military Academy where he served in engineering assignments as well as a professor before leaving the Army to pursue a career in civil engineering. At the start of the Civil War, leading troops from Ohio, he achieved early combat success in western Virginia. In 1862 in the Western Theater, he won the battles of Iuka and Corinth while under the command of Maj. Gen. Ulysses S. Grant. His brusque, outspoken manner and willingness to quarrel openly with superiors caused a professional rivalry with Grant (as well as with Secretary of War Edwin M. Stanton) that would adversely affect Rosecrans' career.

Given command of the Army of the Cumberland, he fought against Confederate Gen. Braxton Bragg at Stones River, and later outmaneuvered him in the brilliant Tullahoma Campaign, driving the Confederates from Middle Tennessee. His strategic movements then caused Bragg to abandon the critical city of Chattanooga, but Rosecrans' pursuit of Bragg ended during the bloody Battle of Chickamauga, where his unfortunately worded order mistakenly opened a gap in the Union line and Rosecrans and a third of his army were swept from the field. Besieged in Chattanooga, Rosecrans was relieved of command by Grant.

Following his humiliating defeat, Rosecrans was reassigned to command the Department of Missouri, where he opposed Price's Raid. After the war, he served in diplomatic and appointed political positions and in 1880 was elected to Congress, representing California.