That was the incident that began the fighting. The Confederacy could have won early if they had advanced into DC following their initial victory. They could have sued for peace if they had prevailed at Gettysburg, which they could possibly have done several times during the engagement, but that was nearly their last chance. The longer the war went on, the dimmer their chances became.
However, it's fair to note that these would constitute "victory" in a diplomatic sense only, and that the underlying cause of the war -- which was expansion of slavery into the new territories -- would still exist. The sectional rivalries would also have been considerably sharpened by the fact of the North having "lost" the war.
Eventually the Civil War would have started afresh, and probably ended with the same utter defeat of the South as actually occurred.