Why was Union victory inevitable after the battle of Gettysburg?

The Battle of Gettysburg (July 1-3 1863) supposed a crushing defeat for the Confederacy. It lost many men, weapons and materiel. Gen. Robert E. Lee had to order a retreat to Virginia again, leaving the North´s territory it had invaded in an attempt to threaten Washington. The South lost the strategic initiative. After Gettysburg, the Southern forces were never able to launch a major offensive again, they kept fighting on the defensive until the end of the conflict That´s why Gettysburg is the most important engagement of the Civil War.

Explanation:

sitting in that one chair where he could get shot at was his worst decision.