I think the Overland Campaign more than anything else speaks to the fighting capacity of the Army of the Potomac. Not to discredit the Army of Northern Virginia, but too often discussions of the Civil War end up praising the bravery and tenacity of an outnumbered band of ragged rebels stubbornly holding on for four years while ignoring the incredible bravery and sacrifice of the Union soldiers. Inevitable Northern victory is often attributed to manpower superiority, but I feel this is far too much of a simplification of the situation.
While Gettysburg was a tremendously important victory for the Union, I feel that it's significance is sometimes overstated. Lee was largely able to make good on his losses after Gettysburg. "General George G. Meade, the federal army's most recent commander, had won his nation's applause by repelling Lee's foray into Northern territory with his victory at Gettysburg in July 1863. But summer, fall, and winter had come and gone, and Meade had accomplished nothing to reap the fruits of his Gettysburg victory. By the spring of 1864, Lee had brought his army back up to strength, and the defensive mind-set of the Union high command had dissipated whatever promise of success Gettysburg had offered the North" (Rhea, Carrying the Flag, 8). It stopped Lee's invasion of the North, but when the campaign season started in 1864 there was Lee sitting square on the other side of the Rapidan same as he had been since 1862. The army was going to once again have to cross that river and get at Lee and offensively defeat him on his own turf - rather than defensively at home. Lee may never have gone on the offensive again after Gettysburg, but to be quite frank the South did not need an offensive victory to win the war, and Lee was simply being ambitious every time he crossed the Potomac. For the third time, the Army of the Potomac was going to have to find the strength to lift itself up and fight, and it was going to have to do it under a new leader whose victories in the West meant little to soldiers who felt that a commander wasn't worth a pinch of salt until he had faced Lee in battle.
The campaign itself was grueling. Though the North certainly had more men than the South, as always in combat the real hard fighting fell on an ever-shrinking number of men at the tip of the spear. And with the North losing more in nearly every battle, that meant more friends and family dying and more emotions to tamp down in order to soldier on. And, whether they knew it or not, ultimate Union victory would depend on whether or not they could prove to the Northern voters that, under Lincoln, the bloodshed could be brought to a close with Union victory.
Furthermore, the number of men is irrelevant if they cannot fight. The draft system was bringing in men, but they certainly were not the cream of the crop (though some certainly went on to be good soldiers). And Grant was stripping the defenses of Washington to get more troops, a sure sign that the army ranks were thinning and that recruitments and drafts were not adequately filling them back up. Sure that was more than the South had to draw on, but at the end of the day it's the tenacity of the guy with the gun that makes the real difference, and Northern soldiers proved time and again on the worst battlefields of the war in 1864 (as they did before and would continue to do after) that they were up to the task. The narrative had always been that one Reb was worth ten Yanks or something to that effect, but the grueling slog of 1864 proved that this was simply not the case. The South may have been worn down to defeat, and superior Northern numbers and industry certainly contributed to this, but I feel that it has often been forgotten that this wearing down was done by the fighting prowess of the Union soldier.
I'm not suggesting that the Southern soldier wasn't brave (though he fought for the most reprehensible of causes) and that he didn't face hardships, just that the Northern soldier was just as brave. To quote Pickett's explanation as to why he was defeated at Gettysburg: "I've always thought the Yankees had something to do with it."
I'll go ahead and hop off my soapbox now : )