Lots of possibilities, especially if you include military actions that weren't officially declared wars.  Still,  these days, you'd be hard-pressed to top the Mexican-American War (1846-1848).

Soldiers from that war to remember:

Ulysses S. Grant
Winfield Scott
George B. McClellan
Ambrose Burnside
Stonewall Jackson
James Longstreet
George Meade
Robert E. Lee
Jefferson Davis.