Final answer:
The defense of Culp's Hill and Pickett's Charge were the two key events during the Battle of Gettysburg that changed the course of the battle and the Civil War, leading to a Union advantage and marking the 'High Watermark of the Confederacy'.
Step-by-step explanation:
The battle of Gettysburg, which took place from July 1 to July 3, 1863, is considered a pivotal turning point in the American Civil War. Two specific events within the battle helped change the course of the battle and the larger war. Firstly, the defense of Culp's Hill, where Union forces repelled a Confederate attack on their defensive positions during the morning of July 3rd.
Secondly, the fateful Pickett's Charge, a massive Confederate frontal assault on the Union center at Cemetery Ridge on the same day. This charge resulted in over half of the 15,000 Confederate soldiers being killed, wounded, or captured, and it marked the 'High Watermark of the Confederacy' as their furthest point of advance into Union territory.