In fact, if you add up the number of firearm-related deaths in America since 1968, that figure is larger than all of the battlefield casualties in all of the wars in American history.
Martin Grandjean, a researcher of contemporary history at the University of Lausanne in Switzerland and a data visualization expert, created a visualization last year showing these figures. The deaths from warfare in the United States between 1775 and 2015 are listed on the left below. The Civil War ranks as the deadliest battle (though Grandjean uses a generous estimate for Civil War casualties) followed by World War II. On the right is an estimate of all firearm-related deaths in the U.S. from 1968 to 2015, the most recent year for which there is data.
Grandjean draws his figures from Mark Shields of PBS, who mentioned the statistic after the Sandy Hook mass shooting, and Nicholas Kristof of the New York Times, who wrote about it following a shooting in Virginia in August 2015. The statement was subsequently fact-verified by Louis Jacobson of Politifact, who gives a great explanation of where the figures come from.