Who was the leading Union general during the American Civil War and later became the eighteenth President of the United States?

The leading Union general during the American Civil War who later became the eighteenth President of the United States was Ulysses S. Grant. Grant played a pivotal role in several key battles and campaigns for the Union, most notably at Vicksburg and the final campaigns against the Confederate Army in Virginia. His military strategies and tenacity contributed significantly to the Union's victory in the Civil War.

After the war, Grant transitioned into politics and was elected as President serving two terms from 1869 to 1877. His presidency focused on Reconstruction efforts and civil rights for freed slaves, though his administration faced numerous challenges, including political corruption and economic difficulties. Grant’s legacy is marked by his leadership during the war and his commitments to civil rights, making him a notable figure in both military and political history.

The other figures mentioned were significant military leaders in their own right but did not serve as the leading general for the Union or become President. Robert E. Lee commanded the Confederate Army, George Meade was notable for his victory at the Battle of Gettysburg, and William Tecumseh Sherman is best known for his "March to the Sea."