At 10:59 a.m. on Nov. 11, 1918, a German machine gun crew fired a burst of rounds at approaching Americans. One bullet struck 23-year-old Pvt. Henry Gunther in the left temple. He died instantly.

World War I is often remembered for mud, trenches, and stalemates. But underneath that ugly surface, the war was really a giant laboratory where new weapons crashed into old tactics and forced the ...