The Battle of Tsushima in 1905. Histories only decisive sea battle fought by modern (steel) battleship fleets.
Battle of Gettysburg
Somme.
Gettysburg
Antietam
Charleston
Antietam
There wasn't a war in the war of 1812 but if you mean battle it is just a matter of opinion
diseases
Gettysburg. And the bloodiest single-day battle was Antietam.
Gettysburg
Chancellorsville
The Battle of Mobile Bay.