The one turning point in the American Civil War that most clearly deserves to be called “seminal” was the issuance of the Emancipation Proclamation.
Up until the time that Lincoln issued the proclamation, the war had simply been a war about preserving the Union. This meant that it was a war only for fairly narrow political purposes, not one that had any clearly noble goal. Once Lincoln issued the proclamation, the war took on a whole different meaning. It was now the war in which people could vow to “die to make men free.”
This was a seminal event because it changed American society forever. It resulted (after the war ended) in the freeing of the slaves. This made the United States a much more just and democratic country.