Background: William McKinley (January 29, 1843 - September 14, 1901) was the 25th President of the United States, serving from March 4, 1897 until his assassination in September 1901, six months into his second term. McKinley led the nation to victory in the Spanish-American War, raised protective tariffs to promote American industry, and maintained the nation on the gold standard in a rejection of free silver (effectively, expansionary monetary policy). McKinley was the last president to have served in the American Civil War, and the only one to have started the war as an enlisted soldier, beginning as a private in the Union Army and ending as a brevet major.
Context: When the Southern states seceded from the Union and the American Civil War began, thousands of men in Ohio volunteered for service. Among them were McKinley and his cousin William McKinley Osbourne, who enlisted as privates in the newly formed Poland Guards in June 1861. The men left for Columbus where they were consolidated with other small units to form the 23rd Ohio Infantry. The men were unhappy to learn that, unlike Ohio's earlier volunteer regiments, they would not be permitted to elect their officers; they would be designated by Ohio's governor, William Dennison. Dennison appointed Colonel William Rosecrans as the commander of the regiment, and the men began training on the outskirts of Columbus. McKinley quickly took to the soldier's life and wrote a series of letters to his hometown newspaper extolling the army and the Union cause. Delays in issuance of uniforms and weapons again brought the men into conflict with their officers, but Major Rutherford B. Hayes convinced them to accept what the government had issued them; his style in dealing with the men impressed McKinley, beginning an association and friendship that would last until Hayes' death in 1893. After a month of training, McKinley and the 23rd Ohio, now led by Colonel Eliakim P. Scammon, set out for western Virginia (today part of West Virginia) in July 1861 as a part of the Kanawha Division. McKinley initially thought Scammon was a martinet, but when the regiment finally saw battle, he came to appreciate the value of their relentless drilling. Their first contact with the enemy came in September when they drove back Confederate troops at Carnifex Ferry in present-day West Virginia. Three days after the battle, McKinley was assigned to duty in the brigade quartermaster office, where he worked both to supply his regiment, and as a clerk. In November, the regiment established winter quarters near Fayetteville (today in West Virginia). McKinley spent the winter substituting for a commissary sergeant who was ill, and in April 1862 he was promoted to that rank. The regiment resumed its advance that spring with Hayes in command (Scammon by then led the brigade) and fought several minor engagements against the rebel forces. That September, McKinley's regiment was called east to reinforce General John Pope's Army of Virginia at the Second Battle of Bull Run. Delayed in passing through Washington, D.C., the 23rd Ohio did not arrive in time for the battle, but joined the Army of the Potomac as it hurried north to cut off Robert E. Lee's Army of Northern Virginia as it advanced into Maryland. The 23rd was the first regiment to encounter the Confederates at the Battle of South Mountain on September 14. After severe losses, Union forces drove back the Confederates and continued to Sharpsburg, Maryland, where they engaged Lee's army at the Battle of Antietam, one of the bloodiest battles of the war. The 23rd was also in the thick of the fighting at Antietam, and McKinley himself came under heavy fire when bringing rations to the men on the line. McKinley's regiment again suffered many casualties, but the Army of the Potomac was victorious and the Confederates retreated into Virginia. The regiment was then detached from the Army of the Potomac and returned by train to western Virginia.
Question: who were there enemy?

Answer:
Confederate troops