April 5 – May 4, 1862
Lasting from April 5 to May 4, 1862, the Siege of Yorktown stalled Union Major General George B. McClellan's Peninsula Campaign for four weeks, enabling the Confederacy to shore up its defenses of Richmond, Virginia.
Within twenty-four hours of the Union's defeat at the Battle of Bull Run I (July 21, 1861), the Lincoln administration called upon George B. McClellan to lead the Union war effort in the East. McClellan spent the first few months of his new command fortifying Washington, DC and reorganizing Federal forces. The Northern public and politicians, however, wanted action. Accordingly, McClellan devised plans for an offensive to capture the Confederate capital at Richmond, Virginia to bring a quick conclusion to the war.
;Although President Lincoln favored an overland assault on Richmond from the Washington area, McClellan developed a more complex strategy. Correctly believing that Confederate leaders anticipated a direct onslaught against the Confederate capital, McClellan proposed to outmaneuver the Rebel army guarding Richmond and to launch an offensive from the southeast, up the Virginia Peninsula.
The Virginia Peninsula is a strip of land in southeastern Virginia that runs from the northwest to the southeast. It is bordered on the north and east by the York River and on the south and west by the James River. Although nearly the entire area was behind Confederate lines early in the war, the Union maintained possession of Fort Monroe at the very tip of the peninsula.
;McClellan planned to use the Union's superior naval resources to transport the Army of the Potomac down the Chesapeake Bay, to disembark at Fort Monroe, and then to move up the peninsula, attacking Richmond from the rear. Implementation of the plan was imperiled in early March when the Confederate Navy launched the ironclad CSS Virginia, posing a menace to Federal control of the waters around Fort Monroe. The Virginia's threat was erased by the appearance of the USS Monitor at the Battle of Hampton Roads, forcing the Virginia's withdrawal on March 9.
McClellan's offensive began on March 17, when he began transporting his army of approximately 120,000 men to Fort Monroe. McClellan arrived on site on April 2, and two days later, the Army of the Potomac began its advance up the peninsula toward Yorktown.
;Confederate officials were not unprepared for McClellan's offensive. In 1861, Major General John Bankhead Magruder, commander of the Army of the Peninsula, began constructing defensive lines across the Virginia Peninsula. The main line, known as the Warwick Line, connected Yorktown on the York River to the headwaters of the Warwick River, and then extended southwest to the confluence of the Warwick and James Rivers.
;As McClellan's men began their advance up the peninsula on April 4, they soon encountered Rebel skirmishers who quickly fell back to the Warwick Line. At that time, Magruder had only twelve thousand soldiers at his disposal to defend the line, which stretched for approximately twelve miles across the peninsula. At a time when the Rebel line could have been easily overrun by McClellan's immense army, Magruder's creativity, however, made up for his lack of manpower. As McClellan's scouts watched, Magruder skillfully moved his men about, behind his defenses, creating the illusion that his force was much larger than it really was. The charade deceived McClellan into estimating the size of Magruder's army to be in excess of forty thousand soldiers. Magruder's theatrics, combined with McClellan's tentative nature, prompted the Union general to order his men to entrench and to prepare for a siege, rather than to overwhelm their adversaries.
;As McClellan's army dug in and constructed siege fortifications, Confederate officials began shifting soldiers from northern Virginia to the Virginia Peninsula. By late April, the Army of the Peninsula was incorporated into the Right Wing of the Army of Northern Virginia and General Joseph E. Johnston arrived to take command of the fifty-six thousand troops that now stood between McClellan and Richmond.
;During the course of the four-week siege, McClellan made one major attempt to break the Confederate line. On April 16, Union forces, commanded by Brigadier-General William F. "Baldy" Smith, crossed the Warwick River in an effort to test the Confederate defenses near Dam No. 1, where Union Brigadier-General Winfield S. Hancock had reported a potential vulnerability. Smith, however, was hampered by McClellan's orders to avoid a general engagement as the Northerners attempted to curb Confederate efforts to shore up their lines. Smith's soldiers succeeded in pushing their adversaries back until a Rebel counterattack drove the Yankees back across the bridge, thus ending the only real Federal attempt to breach the Rebel defenses during the siege.
;For the next two weeks, McClellan continued to amass siege artillery in front of the Warwick Line, while trying unsuccessfully to convince the navy to outflank the Rebels by moving flotillas up the York and James Rivers. By April 22, Johnston realized that his defenders had no chance of withstanding the looming barrage from McClellan's artillery, which eventually totaled 101 siege guns. Consequently, Johnston recommended withdrawing his forces to new defensive lines closer to Richmond. However, Confederate President Jefferson Davis, heeding the advice of his military advisor, Robert E. Lee, ordered Johnston to remain in his current position. Finally, as McClellan was making his final preparations to unleash the full might of his artillery, Johnston's views prevailed. On the night of May 3, Johnston ordered a covering artillery barrage of his own, as his troops and supply trains began falling back toward Richmond. On the next morning, Union Major General Samuel P. Heintzelman boarded an observation balloon, ascended over the Confederate defenses, and then reported to a stunned McClellan that the Rebels had vanished during the night.
;Because McClellan's army held the field after forcing the Rebel defenders to abandon the Warwick Line, the Siege of Yorktown was tactically a Federal victory. In addition, the Union suffered only 182 casualties compared to three hundred for the Confederacy. Still, the month-long delay enabled the Rebels to strengthen their defenses and to concentrate enough forces south of Richmond to protect the Confederate capital and, ultimately, to derail McClellan's Peninsula Campaign.
No Ohio units participated in the Siege of Yorktown.