Let's narrow the field to just the five formal declarations of war by the US, five strokes of the pen...
The War of 1812. ~20,000 Deaths
The Mexican-American War. 13,283 Deaths.
The Spanish-American War. 2,446 Deaths.
WWI (Declaration of War on Germany) 116,516 Deaths.
WWII (Officially Declarations of War on Japan, Germany, Italy, Romania, Blugaria and Hungary) 405,399 Deaths.
Those figures are US casualties only, they don't include the other side. By my count just our official declarations of war led to ~557,644 American deaths. A pen, in the wrong hands, is far more dangerous than a rifle.