Today is the anniversary of the Battle of Yellow Tavern - where Phil Sheridan and the Union cavalry beat the Confederate cavalry, and JEB Stuart was killed. It was an ambiguous victory - Sheridan's raid was big and splashy, but probably did the Army of the Potomac more harm than good, taking away their cavalry, their intelligence, in much the way Stuart's raid during the Gettysburg campaign took away Lee's intelligence, and handicapped him. Sheridan's men got Stuart, but didn't really break the rebel cavalry, who continued to perform their main function (scouting and the like), without the temptation to go riding off on romantic adventures, like this one.
But - but. First - the raid did establish the union cavalry as a military force to be reckoned with. At the beginning of the war, they had been at a terrible disadvantage vs. the south - by 1863, things were starting to come around (Joe Hooker did a lot to turn them into a real force); they fought a number of successful battles in 63, and played a significant part at Gettysburg (holding off the initial advances for a couple hours until the infantry came up, and later driving Stuart off the union rear). And this, in 1864, more or less confirmed the point: they were a force to be reckoned with on the battle field.
But that also indicates something about how Sheridan thought about his cavalry, and something about the changes in the nature of the war. He was an infantry general in the west - and the truth is, he treated his cavalry corps as something very close to a mounted infantry unit. He expected to fight with them - attack; use their mobility to get into positions to cause serious problems to the enemy; and to be able to hold their own in any situation. And they did it, for a number of reasons - some of them technological. Union cavalry were increasingly armed with Spencer rifles - 7 shot repeaters that used metal cartridges - that meant they could put out a massive firepower - and could do it with guns that didn't foul because of wet. The Spencers and other carbines didn't have the range that rifled muskets had - and cavalry had a hard time mustering the mass firepower that usually made the difference in the Ciil War - but they could put out so much lead, they could hold their own.
The power of repeaters was shown in the west - Wilder's Brigade had Spencers, which they used to great effect; at Chickamauga, the 21st Ohio regiment was armed with Colt repeaters, that allowed them to do immense damage. Sheridan was a fighter - he used the cavalry corps almost like one of those mounted infantry units - fast, mobile, and able to outshoot anything they ran into. It didn't really bear fruit in early 1864 - but it became more and more effective as the year went along, and by the spring of 65, Sheridan would be able to use his troopers, and indeed, much of the union infantry, in that kind of mobile, high firepower way. It didn't quite unbalance the Civil war - there weren't enough of the repeaters around, and the tactics weren't there yet. But it is another of the things that hint at the future. Movement and firepower - well - a future that didn't quite come into being until the internal combustion engine replaced the horse. But the idea was there...