Robert E. Lee's withdraw from the Fredericksburg on June 3rd had been observed and reported to Joe Hooker, the Federal commander. Hooker ordered John Sedgwick to advance with his VI Corps and verify these reports. If Lee had indeed abandoned his lines, Hooker would have to move to maintain contact.
Sedgwick's men advanced on the morning of the 5th, 150 years ago today. They encountered Confederate skirmishers from A. P. Hill's Third Corps, which Lee had left behind to foil Union attempts at pursuit. These rebel skirmishers were able to beat back Federal attempts to cross the Rappahannock River at Deep Run. The Union brought up artillery, but its fire still could not drive off the Confederates.
Finally the 26th New Jersey and 5th Vermont loaded into pontoon boats to cross the river. They were able to rush across the river and land on the opposite shore. Charging forward, they captured the rifle pits and 35 prisoners. They pushed forward, and encountered more, and stronger, rebel detachments. A fierce skirmish developed, and finally the Confederates were able to drive the Union troops back across the river. The Confederates lost 6 killed in addition to the 35 captured. The Federals lost over 50 men.
The next day Hill fell back to follow in Lee's tracks. But this small skirmish had convinced Sedgwick and Hooker that Lee was still in his Fredericksburg lines in full force. It would be several more days until another reconnaissance party was sent forward that discovered the truth.