It's generally agreed Antietam/Sharpsburg was inconclusive on the field, but forced the Confederates to retreat and was therefore strategically a Union victory. Not a complete victory, due to the incompetence of McClellan and Burnside, but enough of one to allow Lincoln to issue the Emancipation Proclamation. This ended southern hopes of British intervention, and therefore for practical purposes ended their hopes of winning the war.
That's about as decisive a strategic victory as one could hope for.