The event that prompted the Union forces to take a defensive position at Gettysburg was a surprise Confederate attack.

On July 1, as Confederate troops moved into the town, Union forces began to engage in a series of skirmishes. Confronted with unanticipated hostility and understanding the strategic significance of the elevated terrain surrounding Gettysburg, Union commander George Meade made the vital choice to set up defensive positions. This action enabled the Union to strengthen its lines, preparing for a lengthy and critical battle in the days to come.