Question: What battle was fought by Greenwich Union soldiers, in which General Mead's Army of the Potomac defeated General Lee's North Virginia Army resulting in the largest number of casualties in a single war?

Context paragraphs:
Battle of Appomattox Court House: The Battle of Appomattox Court House (Virginia, U.S.), fought on the morning of April 9th, 1865, was one of the last battles of the American Civil War (1861-1865).  It was the final engagement of Confederate States Army general Robert E. Lee's Army of Northern Virginia before it surrendered to the Union Army / Army of the Potomac under Lt. Gen. and General-in-Chief Ulysses S. Grant.  Lee, having abandoned the Confederate capital of Richmond, Virginia, after the ten-month Siege of Petersburg and Richmond, retreated west, hoping to join his army with the remaining Confederate forces in North Carolina of the Army of Tennessee under Gen. Joseph E. Johnston.  Union infantry and cavalry forces under Gen. Phillip Sheridan pursued and cut off the Confederates' retreat at the central Virginia village of Appomattox Court House.  Lee launched an last-ditch attack to break through the Union forces to his front, assuming the Union force consisted entirely of lightly armed cavalry.  When he realized that the cavalry was now backed up by two corps of Federal infantry, he had no choice but to surrender with his further avenue of retreat and escape now cut off.

Battle of Amelia Springs: The Battle of Amelia Springs, Virginia was an engagement between the Union Army (Army of the Shenandoah, Army of the Potomac and Army of the James) and Confederate Army of Northern Virginia that occurred on April 5, 1865 during the Appomattox Campaign of the American Civil War.  It was followed by a second rear guard action near the same location on the night of April 5, 1865 and morning of April 6, 1865 during the Union Army pursuit of the Confederate forces (Army of Northern Virginia and Richmond local defense forces) which were fleeing westward after the fall of Petersburg and Richmond, Virginia at the Third Battle of Petersburg (sometimes shown as the Breakthrough at Petersburg) on April 2, 1865.  The actions took place just prior to the Battle of Sailor's Creek (sometimes shown as "Sayler's Creek") on April 6, 1865.  That battle would be the last major engagement between the Union Army under the overall direction of Union General-in-Chief, Lt. Gen. Ulysses S. Grant, and General Robert E. Lee's Army of Northern Virginia before that Confederate army's surrender at Appomattox Court House, Virginia on April 9, 1865.

Greenwich in the American Civil War: The town of Greenwich, Connecticut, contributed 437 men to twenty-six Connecticut regiments during the American Civil War.  Greenwich soldiers fought in almost every major Union campaign, including Bull Run, Gettysburg and the siege of Petersburg.  Approximately half of the Greenwich soldiers served in two infantry regiments, the 10th Connecticut Infantry and 17th Connecticut Infantry.

Battle of White Oak Road: The Battle of White Oak Road, also known as The Battle of Hatcher’s Run, Gravelly Run, Boydton Plank Road, White Oak Ridge was fought on March 31, 1865, during the American Civil War at the end of the Richmond-Petersburg Campaign and in the beginning stage of the Appomattox Campaign.  Along with the Battle of Dinwiddie Court House which was fought simultaneously on March 31, the battle involved the last offensive action by General Robert E. Lee's Confederate Army of Northern Virginia to stop the progress of Lieutenant General Ulysses S. Grant's Union Army (Army of the Potomac, Army of the Shenandoah and Army of the James).  Grant's forces were moving to cut the remaining Confederate supply lines and to force the Confederates to extend their defensive lines at Petersburg, Virginia and Richmond, Virginia to the breaking point, if not to force them into a decisive open field battle.

Battle of Gettysburg: 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.

Camp Douglas (Chicago): Camp Douglas, in Chicago, Illinois, sometimes described as "The North's Andersonville" was the largest Union Army prisoner-of-war camps for Confederate soldiers taken prisoner during the American Civil War.  Based south of the city on the prairie, it was also used as a training and detention camp for Union soldiers.  The Union Army first used the camp in 1861 as an organizational and training camp for volunteer regiments.  It became a prisoner-of-war camp in early 1862.  Later in 1862 the Union Army again used Camp Douglas as a training camp.  In the fall of 1862, the Union Army used the facility as a detention camp for paroled Confederate prisoners (these were Union soldiers who had been captured by the Confederacy and sent North under an agreement that they would be held temporarily while formal prisoner exchanges were worked out).

Battle of Chancellorsville: The Battle of Chancellorsville was a major battle of the American Civil War (1861–1865), and the principal engagement of the Chancellorsville Campaign.  It was fought from April 30 to May 6, 1863, in Spotsylvania County, Virginia, near the village of Chancellorsville.  Two related battles were fought nearby on May 3 in the vicinity of Fredericksburg.  The campaign pitted Union Army Maj. Gen. Joseph Hooker's Army of the Potomac against an army less than half its size, General Robert E. Lee's Confederate Army of Northern Virginia.  Chancellorsville is known as Lee's "perfect battle" because his risky decision to divide his army in the presence of a much larger enemy force resulted in a significant Confederate victory.  The victory, a product of Lee's audacity and Hooker's timid decision making, was tempered by heavy casualties, including Lt. Gen. Thomas J. "Stonewall" Jackson.  Jackson was hit by friendly fire, requiring his left arm to be amputated; he died of pneumonia eight days later, a loss that Lee likened to losing his right arm.

Battle of Aquia Creek: The Battle of Aquia Creek was an exchange of cannon fire between Union Navy gunboats and Confederate shore batteries on the Potomac River at its confluence with Aquia Creek in Stafford County, Virginia.  The battle took place from May 29, 1861 to June 1, 1861 during the early days of the American Civil War.  The Confederates set up several shore batteries to block Union military and commercial vessels from moving in the Chesapeake Bay and along the lower Potomac River as well as for defensive purposes.  The battery at Aquia also was intended to protect the railroad terminal at that location.  The Union forces sought to destroy or remove these batteries as part of the effort to blockade Confederate States coastal and Chesapeake Bay ports.  The battle was tactically inconclusive.  Each side inflicted little damage and no serious casualties on the other.  The Union vessels were unable to dislodge the Confederates from their positions or to inflict serious casualties on their garrisons or serious damage to their batteries.  The Confederates manning the batteries were unable to inflict serious casualties on the Union sailors or cause serious damage to the Union vessels.  Soon after the battle, on Sunday, July 7, 1861, the Confederates first used naval mines, unsuccessfully, off the Aquia Landing batteries.  The Confederates ultimately abandoned the batteries on March 9, 1862 as they moved forces to meet the threat created by the Union Army's Peninsula Campaign.  The U. S. National Park Service includes this engagement in its list of 384 principal battles of the American Civil War.

Battle of Fredericksburg: The Battle of Fredericksburg was fought December 11–15, 1862, in and around Fredericksburg, Virginia, between General Robert E. Lee's Confederate Army of Northern Virginia and the Union Army of the Potomac, commanded by Major General Ambrose Burnside, as part of the American Civil War.  The Union Army's futile frontal attacks on December 13 against entrenched Confederate defenders on the heights behind the city are remembered as one of the most one-sided battles of the war, with Union casualties more than three times as heavy as those suffered by the Confederates.  A visitor to the battlefield described the battle to U.S. President Abraham Lincoln as a "butchery."

Gettysburg Campaign: 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.

Supporting paragraphs: Greenwich in the American Civil War Battle of Gettysburg
Answer: Battle of Gettysburg