Today is the 147th anniversary of the infamous (from the Union perspective) Battle of Fredericksburg. This made me think, what was the most decisive victory (or defeat depending on your point of view) in the war? Open to any answers.
Depends on what you consider decisive.
Rather than simply what were the most one-sided victories, I define a decisive battle/campaign as one which had an important permanent military and/or political impact on the outcome of the war.
1) Fort Henry/Fort Donelson -- Broke the rebel defense line in Kentucky, opened the path to Memphis, Nashville, and Knoxville, and made "Unconditional Surrender" Grant a man to be considered.
2) Pea Ridge -- ended rebel chances for a "third front" anchored by a Confederate Missouri.
4) Glorieta Pass -- ended rebel ambitions for conquest in the southwest that would extend the Confederacy to the Pacific Ocean and push it northward into Colorado and Utah.
4) New Orleans -- deprived the Confederacy of its largest city, its most important port, a major manufacturing center, and began the opening of the Mississippi River to Union control.
5) Antietam -- paved the way for the Emancipation Proclamation thus helping deprive the Confederacy its slave labor source while giving the Union a new manpower supply, pleased the Republican radicals while not alienating the War Democrats, and made it far more difficult for the British -- and therefore the French -- to consider acting on behlf of the Confederacy.
6) Vicksburg -- deprived the eastern Confederacy of the "hog and hominy" of the Trans-Mississippi, accomplished the war aim highly important to the northwest of making the Mississippi River blue water, and reinforced Grant's stature as the man of the war.
7) Chattanooga -- effectively ended Confederate advances in the Cis-Mississippi with any chance of real success (Hood's Nashville campaign had no such chance), and ensured that Grant would be brought to the command that would mean an end to the rebellion.
8) The trifecta of Atlanta/Mobile Bay/Cedar Creek -- ensured the re-election of Lincoln and the continuance of the war to northern victory.
9) Overland Campaign/Petersburg-- pinned Lee and the AoNVa in place and made its destruction at Appomattox inevitable.