Question: What  battle of the Civil War involved the largest number of casualties of the entire war and is often described as the war's turning point, Second Battle of Bull Run or Battle of Gettysburg?

Context articles: Irvin McDowell
Henry Warner Slocum
Gettysburg Campaign
Take Command (video game)
1st Minnesota Volunteer Infantry
Turning point of the American Civil War
Second Battle of Bull Run
Eric J. Wittenberg
Manassas National Battlefield Park
Battle of Gettysburg

Irvin McDowell (October 15, 1818 – May 4, 1885) was a career American army officer.
 He is best known for his defeat in the First Battle of Bull Run, the first large-scale battle of the American Civil War.
 In 1862, he was given command of the I Corps of the Army of the Potomac.
 He fought unsuccessfully against Stonewall Jackson's troops during the Valley Campaign of 1862, and was blamed for contributing to the defeat of United States troops at the Second Battle of Bull Run in August.
Henry Warner Slocum, Sr. (September 24, 1827 – April 14, 1894), was a Union general during the American Civil War and later served in the United States House of Representatives from New York.
 During the war, he was one of the youngest major generals in the Army and fought numerous major battles in the Eastern Theater and in Georgia and the Carolinas.
 While commanding a regiment, a brigade, a division, and a corps in the Army of the Potomac, he saw action at Bull Run, the Peninsula, South Mountain, Antietam, Chancellorsville, and Harpers’ Ferry.
 At Gettysburg, he was the senior Union General in the Field.
 During the battle, he held the Union right from Culp’s Hill to across the Baltimore Pike.
 His successful defense of Culp’s Hill was crucial to the Union victory at Gettysburg.
 After the fall of Vicksburg, Slocum was appointed military commander of the district.
 Slocum participated in the Atlanta campaign and was the first commander to enter the city on September 2, 1864.
 He then served as occupation commander of Atlanta.
 Slocum was appointed the commander of the left wing of Sherman’s march through Georgia and the Carolinas, commanding the XIV and XX Corps, comprising the Army of Georgia.
 During this campaign, he captured the capitol of Georgia, Milledgeville, and the seaport of Savannah.
 In the Carolinas campaign, Slocum’s army saw victories in the battles of Averasborough and Bentonville, North Carolina.
 The March to the Sea and the Carolinas campaign were crucial to the overall Union victory in the Civil War.
 After the surrender of Confederate forces, Slocum was given command of the Department of Mississippi.
 Slocum declined an appointment in the postwar Army.
 He was a successful political leader, businessman and railroad developer.
The Gettysburg Campaign was a military invasion of Pennsylvania by the main Confederate army under General Robert E. Lee in summer 1863.
 The Union won a decisive victory at Gettysburg July 1–3, with heavy casualties on both sides.
 Lee managed to escape back to Virginia with most of his army.
 It was a turning point in the American Civil War, with Lee increasingly pushed back toward Richmond until his surrender in April 1865.
 After his victory in the Battle of Chancellorsville, Lee's Army of Northern Virginia moved north for a massive raid designed to obtain desperately needed supplies, to undermine civilian morale in the North, and to encourage anti-war elements.
 The Union Army of the Potomac was commanded by Maj. Gen. Joseph Hooker and then (from June 28) by Maj. Gen. George G. Meade.
Take Command is a series of real-time tactics computer games by MadMinute Games.
 As of August 2006 the series consist of two games, Take Command: Bull Run (a.k.a.
 The History Channel Civil War The Battle of Bull Run Take Command: 1861) (2004) and Take Command - 2nd Manassas (2006).
 The games are real-time wargames depicting some of the major battles of the American Civil War.
 The developers describe the games as "real-time combat simulators".
 The first game was released under the "Activision Value" brand, which is Activision's budget line.
 The second game is released through Paradox, a Swedish publisher that specializes in strategy games.
 A third game, based on the Battle of Shiloh, was said to be in development according to the instruction manual for 2nd Manassas, but has likely been discontinued.
The First Minnesota Volunteer Infantry Regiment mustered for a three-year term (1861-1864) in the Union Army at the outset of the American Civil War when the prevailing enlistment period was three months.
 During offensive movements, it sustained high percentages of casualties at the Battles of First Bull Run (20%) and Antietam (28%) and a catastrophic 82% at the Battle of Gettysburg.
 It is most noted for its service on the second day at Gettysburg.
There is widespread disagreement among historians about the turning point of the American Civil War.
 A turning point in this context is an event that occurred during the conflict after which most modern scholars would agree that the eventual outcome was inevitable.
 While the Battle of Gettysburg in July 1863 is the event most widely cited as the military climax of the American Civil War (often in combination with the Siege of Vicksburg, which concluded a day later), there were several other decisive battles and events throughout the war which have been proposed as turning points.
 These events are presented here in chronological order.
 Only the positive arguments for each are given.
The Second Battle of Bull Run or Battle of Second Manassas was fought August 28–30, 1862 in Prince William County, Virginia, as part of the American Civil War.
 It was the culmination of the Northern Virginia Campaign waged by Confederate Gen. Robert E. Lee's Army of Northern Virginia against Union Maj. Gen. John Pope's Army of Virginia, and a battle of much larger scale and numbers than the First Battle of Bull Run (or "First Manassas") fought on July 21, 1861 on the same ground.
Eric J. Wittenberg (born March 26, 1961) is an American Civil War (Civil War) historian, author, lecturer, tour guide and battlefield preservationist.
 He is a practicing attorney in downtown Columbus, Ohio.
 His published works have focused especially on the Civil War cavalryman and the cavalry battles of the Civil War, with emphasis on the Army of the Potomac's Cavalry Corps.
 His first book, "Gettysburg's Forgotten Cavalry Actions", was chosen as the best new work addressing the Battle of Gettysburg in 1998, winning the Robert E. Lee Civil War Roundtable of Central New Jersey's Bachelder-Coddington Award.
 The second edition of this book, published in 2011, won the U. S. Army Historical Foundation's Distinguished Writing Award for that year's best reprint.
 In 2015, his book "The Devil's to Pay: John Buford at Gettysburg" won the Gettysburg Civil War Roundtable's 2015 Book Award.
 He was a member of the Governor of Ohio’s Advisory Commission on the Sesquicentennial of the Civil War and has been active with several Civil War battlefield preservation organizations.
 He and his wife Susan Skilken Wittenberg reside on the east side of Columbus, Ohio.
Manassas National Battlefield Park, located north of Manassas, in Prince William County, Virginia, preserves the site of two major American Civil War battles: the First Battle of Bull Run on July 21, 1861, and the Second Battle of Bull Run which was fought between August 28 and August 30, 1862 (also known as the "First Battle of Manassas" and the "Second Battle of Manassas", respectively).
 The peaceful Virginia countryside bore witness to clashes between the armies of the North (Union) and the South (Confederacy), and it was there that Confederate General Thomas J. Jackson acquired his nickname "Stonewall."
The Battle of Gettysburg ( , with an sound) was fought July 1–3, 1863, in and around the town of Gettysburg, Pennsylvania, by Union and Confederate forces during the American Civil War.
 The battle involved the largest number of casualties of the entire war and is often described as the war's turning point.
 Union Maj. Gen. George Meade's Army of the Potomac defeated attacks by Confederate Gen. Robert E. Lee's Army of Northern Virginia, halting Lee's invasion of the North.