The first failure was caused by our not following up the success gained over Hill's corps on the morning of the 6th, as before described: the second, when fires caused by that battle drove Anderson to make a march during the night of the 7th-8th which he was ordered to commence on the morning of the 8th.
But accident often decides the fate of battle.
Sheridan's cavalry had had considerable fighting during the afternoon of the 7th, lasting at Todd's Tavern until after night, with the field his at the close.

He issued the necessary orders for seizing Spottsylvania and holding the bridge over the Po River, which Lee's troops would have to cross to get to Spottsylvania.

But Meade changed Sheridan's orders to Merritt--who was holding the bridge--on his arrival at Todd's Tavern, and thereby left the road free for Anderson when he came up.

Wilson, who was ordered to seize the town, did so, with his division of cavalry; but he could not hold it against the Confederate corps which had not been detained at the crossing of the Po, as it would have been but for the unfortunate change in Merritt's orders.

Had he been permitted to execute the orders Sheridan gave him, he would have been guarding with two brigades of cavalry the bridge over the Po River which Anderson had to cross, and must have detained him long enough to enable Warren to reinforce Wilson and hold the town.
Anderson soon intrenched himself--if indeed the intrenchments were not already made--immediately across Warren's front.