From the winter of 1861 through the spring of 1863 it launched a series of invasions of northern Virginia. Each of these invasions ended in decisive defeat, usually at the hands of the Confederate Army of Northern Virginia and its commander Robert E. Lee. It technically won a tactical victory against the ANV during Lee's Maryland Campaign in the autumn of 1862, but the army's commander, the ineffective George McClellan, passed up opportunities to make the victory decisive.
In the summer of 1863, Lee launched a second invasion of the loyal states. Bare days before the Army of the Potomac was forced to offer battle (at Gettysburg, Pennsylvania) Abraham Lincoln replaced the army's commander, General Joseph Hooker, with General George Meade. Meade won a decisive victory against Lee and drove him back into Virginia, never to threaten the loyal states again.
In the spring of 1864 the newly promoted General-in-Chief Ulysses S. Grant personally led an invasion of Virginia. The Army of the Potomac was the main force of this invasion; despite having had some units transferred to other commands since Gettysburg, it remained the US's largest army. Meade continued to command the Army but it was Grant who was ultimately responsible for field command decisions during this campaign.
The Overland Campaign saw Union forces cross to the southern shore of the Rapidan River and lay siege to the city of Petersburg. After nine months of siege warfare, the Army of the Potomac broke through and put its archrival the Army of Northern Virginia on the run, finally attaining its longterm objective of capturing the Rebel capital of Richmond. A short while later, Army of the Potomac forces ran down the Army of Northern Virginia and forced Lee to capitulate at the town of Appomattox, ending the war in the Eastern Theater.
Army of the Potomac in The Guns of the South
On the May 4, 1864, the Army of the Potomac crossed the Rapidan and attempted to pass through the Wilderness to threaten the Army of Northern Virginia, commanded by Robert E. Lee. Andries Rhoodie, leader of the mysterious "Rivington Men", correctly informed Lee that General Ulysses S. Grant was not really aiming to take Richmond if he won the battle in the Wilderness; instead, Grant was after Lee and the Army of Northern Virginia. After three years of war, the Army of the Potomac held an unquestionable advantage in firepower, numbers, and logistical support and was likely to win the coming campaign. No one on the Union side knew, however, how drastically the situation had suddenly changed thanks to the AWB's efforts.
Wielding an extraordinarily lethal new rifle, the AK-47, the ANV was able to defeat the Army of the Potomac in the Wilderness while inflicting heavy casualties. The Army of the Potomac retreated on May 6 to the small town of Bealeton, Virginia where it suffered a second major defeat. The New York Times reported "a loss of upwards of 40,000 men in the two battles and absolutely nothing gained." Entire regiments were destroyed in the fighting- so many that II Corps under General Winfield Scott Hancock ceased to exist as a functioning unit. Casualties were particularly extreme among black soldiers and their white officers; the division of United States Colored Troops commanded by General Edward Ferrero was virtually annihilated.
Unable to advance further into Virginia, the Army then retreated towards Washington City, attempting to reorganize itself, but Lee attacked immediately, and the Army of Northern Virginia overran and captured the city in a daring night assault. General Ulysses S. Grant tried to bring surviving elements of the Army of the Potomac back into Washington to save the city, but Confederate artillery dropped key bridges into the river and forced them to sit and watch as Washington fell. The 1864 campaign, and with it the war, ended in disaster and defeat for the Army of the Potomac, an ignominious fate after so many sacrifices in four years of hard fighting.
At the time of the Confederacy's sudden reversal of fortunes, the Army of the Potomac was under the command of General George Meade, though General-in-Chief Grant had attached himself to it and was in command of the campaign, which included the IX Corps as well as the four corps of the Army of the Potomac.
Army of the Potomac in Southern Victory
When the Army of the Potomac was organized in the winter of 1861, it was the largest army ever seen in the history of the US. However, its commander, George McClellan, was extremely reluctant to commit it to action and the army mostly stayed within the defenses of Washington, DC throughout the spring of 1862. McClellan eventually campaigned against Richmond by marching up the Peninsula in April 1862, but McClellan's timidity in the face of resistance of a much smaller army caused the Army of the Potomac to be driven away from Richmond.
In September 1862, it was the Army of the Potomac's turn to defend its capital when Confederate General Robert E. Lee invaded the US. Due partly to poor Union intelligence and partly to McClellan's own incompetence, Lee was able to steal a march on the Army of the Potomac and threaten Philadelphia. In desperation, and against the advice of his subordinates such as Ambrose Burnside, McClellan offered battle on the unfavorable terrain of Camp Hill, Pennsylvania. The Army of the Potomac was routed and completely destroyed in the battle. Lee's Army of Northern Virginia captured Philadelphia, and the War of Secession came to an end with the Union cause in ruins as Britain and France extended diplomatic recognition to the Confederacy.
When the Second Mexican War began in 1881, the Army of the Potomac was reformed, and hastily sent into action in order to outflank the forts that over looked Washington DC. However, Confederate General Thomas Jackson was quick to move, while his Union counterpart was not, quickly attacking the Army at Winchester, and defeating them. The Army of the Potomac was routed and fled all the way back to Harpers Ferry. It did not engage in any more fighting for the rest of the war, as all reinforcements were sent to the campaign in Kentucky.
In May of 1882, after General Jackson started shipping men to bolster the Army of Northern Virginia, the Army of the Potomac finally began to received reinforcements. Although they were more than prepared to fight, the outcome was unclear as the war had gone so badly for the US.
The war ended before any major action could take place.