Question: The Battle of Cedar Mountain, also known as Slaughter's Mountain or Cedar Run, took place on August 9, 1862, in Culpeper County, Virginia, as part of the American Civil War, Union forces under which Maj. Gen., that was an American politician from Massachusetts and a Union general during the Civil War, attacked Confederate forces under Maj. Gen. Thomas J. "Stonewall" Jackson near Cedar Mountain?
Context: The Battle of Lookout Mountain was fought November 24, 1863, as part of the Chattanooga Campaign of the American Civil War.  Union forces under Maj. Gen. Joseph Hooker assaulted Lookout Mountain, Chattanooga, Tennessee, and defeated Confederate forces commanded by Maj. Gen. Carter L. Stevenson.  Lookout Mountain was one engagement in the Chattanooga battles between Maj. Gen. Ulysses S. Grant's Military Division of the Mississippi and the Confederate Army of Tennessee, commanded by Gen. Braxton Bragg.  It drove in the Confederate left flank and allowed Hooker's men to assist in the Battle of Missionary Ridge the following day, which routed Bragg's army, lifting the siege of Union forces in Chattanooga, and opening the gateway into the Deep South. Cedar Mountain, also known as Slaughter Mountain, is a piedmont monadnock in Culpeper County, Virginia.  The 800 ft ridge is 7 mi south of the town of Culpeper and just to the northwest of the northern tip of the Southwest Mountains at Clark Mountain.  The mountain was the site of the Battle of Cedar Mountain in 1862 during the Northern Virginia Campaign of the American Civil War. The Battle of Monocacy (also known as Monocacy Junction) was fought on July 9, 1864, approximately 6 mi from Frederick, Maryland, as part of the Valley Campaigns of 1864 during the American Civil War.  Confederate forces under Lt. Gen. Jubal A. Early defeated Union forces under Maj. Gen. Lew Wallace.  The battle was part of Early's raid through the Shenandoah Valley and into Maryland in an attempt to divert Union forces away from Gen. Robert E. Lee's army under siege at Petersburg, Virginia.  The battle was the northernmost Confederate victory of the war.  While the Union troops retreated to Baltimore, Maryland, the Confederates continued toward Washington, D.C. , but the battle at Monocacy delayed Early's march for a day, allowing time for Union reinforcements to arrive in the Union capital.  The Confederates launched an attack on Washington on July 12 at the Battle of Fort Stevens, but were unsuccessful and retreated to Virginia. Nathaniel Prentice (or Prentiss) Banks (January 30, 1816 – September 1, 1894) was an American politician from Massachusetts and a Union general during the Civil War. The Battle of Savage's Station took place on June 29, 1862, in Henrico County, Virginia, as the fourth of the Seven Days Battles (Peninsula Campaign) of the American Civil War.  The main body of the Union Army of the Potomac began a general withdrawal toward the James River.  Confederate Brig. Gen. John B. Magruder pursued along the railroad and the Williamsburg Road and struck Maj. Gen. Edwin Vose Sumner's II Corps (the Union rearguard) with three brigades near Savage's Station, while Maj. Gen. Thomas J. "Stonewall" Jackson's divisions were stalled north of the Chickahominy River.  Union forces continued to withdraw across White Oak Swamp, abandoning supplies and more than 2,500 wounded soldiers in a field hospital. The Knoxville Campaign was a series of American Civil War battles and maneuvers in East Tennessee during the fall of 1863 designed to secure control of the city of Knoxville and with it the railroad that linked the Confederacy east and west.  Union forces under Maj. Gen. Ambrose Burnside occupied Knoxville, Tennessee, and Confederate forces under Lt. Gen. James Longstreet were detached from Gen. Braxton Bragg's Army of Tennessee at Chattanooga to prevent Burnside's reinforcement of the besieged Union forces there.  Ultimately, Longstreet's own siege of Knoxville ended when Union Maj. Gen. William Tecumseh Sherman led elements of the Army of the Tennessee and other troops to Burnside's relief after Union troops had broken the Confederate siege of Chattanooga.  Although Longstreet was one of Gen. Robert E. Lee's best corps commanders in the East, he was unsuccessful in his role as an independent commander in the West and accomplished little in the Knoxville Campaign. The Battle of Beaver Dam Creek, also known as the Battle of Mechanicsville or Ellerson's Mill, took place on June 26, 1862, in Hanover County, Virginia, as the first major engagement of the Seven Days Battles during the Peninsula Campaign of the American Civil War.  It was the start of Confederate General Robert E. Lee's counter-offensive against the Union Army of the Potomac, under Maj. Gen. George B. McClellan, which threatened the Confederate capital of Richmond.  Lee attempted to turn the Union right flank, north of the Chickahominy River, with troops under Maj. Gen. Thomas J. "Stonewall" Jackson, but Jackson failed to arrive on time.  Instead, Maj. Gen. A.P. Hill threw his division, reinforced by one of Maj. Gen. D.H. Hill's brigades, into a series of futile assaults against Brig. Gen. Fitz John Porter's V Corps, which occupied defensive works behind Beaver Dam Creek.  Confederate attacks were driven back with heavy casualties.  Porter withdrew his corps safely to Gaines Mill. The Battle of Cedar Mountain, also known as Slaughter's Mountain or Cedar Run, took place on August 9, 1862, in Culpeper County, Virginia, as part of the American Civil War.  Union forces under Maj. Gen. Nathaniel P. Banks attacked Confederate forces under Maj. Gen. Thomas J. "Stonewall" Jackson near Cedar Mountain as the Confederates marched on Culpeper Court House to forestall a Union advance into central Virginia.  After nearly being driven from the field in the early part of the battle, a Confederate counterattack broke the Union lines resulting in a Confederate victory.  The battle was the first combat of the Northern Virginia Campaign. The Battle of Thoroughfare Gap, also known as Chapman's Mill, took place on August 28, 1862, in Fauquier County and Prince William County, Virginia, as part of the Northern Virginia Campaign of the American Civil War.  Confederate forces under Maj. Gen. James Longstreet successfully drove back Union forces under Brig. Gen. James B. Ricketts and Col. Percy Wyndham, allowing his corps to unite with that of Thomas J. "Stonewall" Jackson prior to the Second Battle of Bull Run (Second Manassas). The Battle of Gaines's Mill, sometimes known as the First Battle of Cold Harbor or the Battle of Chickahominy River, took place on June 27, 1862, in Hanover County, Virginia, as the third of the Seven Days Battles (Peninsula Campaign) of the American Civil War.  Following the inconclusive Battle of Beaver Dam Creek (Mechanicsville) the previous day, Confederate General Robert E. Lee renewed his attacks against the right flank of the Union Army, relatively isolated on the northern side of the Chickahominy River.  There, Brig. Gen. Fitz John Porter's V Corps had established a strong defensive line behind Boatswain's Swamp.  Lee's force was destined to launch the largest Confederate attack of the war, about 57,000 men in six divisions.  Porter's reinforced V Corps held fast for the afternoon as the Confederates attacked in a disjointed manner, first with the division of Maj. Gen. A.P. Hill, then Maj. Gen. Richard S. Ewell, suffering heavy casualties.  The arrival of Maj. Gen. Stonewall Jackson's command was delayed, preventing the full concentration of Confederate force before Porter received some reinforcements from the VI Corps.
Answer: Nathaniel Prentice (or Prentiss) Banks