Question

at the start of the battle of gettysburg, the union army
drove the confederates out of pennsylvania.
was forced into a defensive position.
broke through the confederate armys line.
fought the confederate army to a stand - off

Explanation:

At the start of the Battle of Gettysburg, the Confederate forces moved into Pennsylvania. The Union army was caught off - guard and was forced into a defensive position to meet the Confederate advance.

Answer:

was forced into a defensive position.