What do Americans owe fellow citizens who gave their lives in war? Memorial Day exists to prompt each of us to pause and reflect on that question. My answer this year is that we should honor their sacrifice by applying lessons of wars past in order to avoid unnecessary wars that could send future generations to premature graves.

The term “unnecessary war” will remind most readers of the invasion of Iraq in 2003. Fourteen years later, that war has claimed the lives of more than 4,000 young Americans, as well as tens of thousands of Iraqis. But as the greatest statesman of the 20th century taught us, the grand reapers of that century—World War I and World War II—were also, properly understood, “unnecessary wars.” And both hold trenchant lessons for us today as we face dual challenges from a rising China and a resurgent Russia.

One hundred years ago today, Americans were just arriving in Europe to join the fight. How could the assassination of an archduke in June 1914 have sparked a conflagration so devastating that it required historians to create an entirely new category—world war? In part, the great states of Europe had become so accustomed to peace among themselves that they had forgotten the horrors of the Napoleonic Wars a century earlier. In part, they had become entangled in an array of alliances that, as Henry Kissinger has described it, created a “doomsday machine.” Actions by third parties, like the terrorist who assassinated Austro-Hungarian Archduke Franz Ferdinand, might have been manageable, but under these conditions triggered reactions that produced a catastrophic outcome.

When the war ended in 1918, all of the major actors had lost what they cared about most: the Austro-Hungarian emperor ousted and his empire dissolved; the Russian czar overthrown by the Bolsheviks; the kaiser dismissed; France bled for a generation; Britain shorn of its treasure and youth. 

So why did statesmen not see more clearly and act more boldly? Unfortunately, Germany’s greatest statesman, Otto von Bismarck, had been dismissed by the kaiser. Without him, the kaiser was like a balloon on a tether, fluttering in the wind. British Foreign Minister Edward Grey had outlined a proposal to punish the Serbs that the major parties would have accepted without war. But he and leaders in other capitals spent more days in July and August on vacation than they did attempting to resolve the crisis. When the war came, it extinguished nearly 25 million lives, 116,000 of them young Americans now buried in graves from Massachusetts to Meuse-Argonne. ...