1870-71: Franco-Prussian War, 433K dead 1914-1918: WWI, 17.6M dead, including 116K Americans 1939-1945: WWII, 54M dead in Europe, including 277K Americans 1949: NATO founded 1949-Present: No major European wars 2018: "NATO's stupid. What does it do for us anyway?"
Those were usually less voluntary. And often involved small principalities or city-states, rather than sovereign nations. You're right that something similar has happened before. But that's why I wrote "rare" instead of "unprecedented."