Why was Union victory inevitable after the battle of Gettysburg?

Because

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.