Imagine you're a military officer in World War I. Armies have grown so large, you can no longer communicate just by the sound of your voice or the wave of your hand. You need to synchronize movements of troops and artillery, far and wide.

You need a wristwatch.

"The entire process of using artillery to protect folks in the trenches, as they advanced, was an elaborately timed, choreographed forward motion," said Carlene Stephens of the Smithsonian's National Museum of American History, who's showing me a 1917 Elgin watch, the kind worn by many soldiers.

The U.S. sat on the sidelines for nearly three years, as World War I raged in Europe. But after renewed submarine attacks by German submarines on civilian vessels, including American ships, President Woodrow Wilson called for war and Congress backed him on April 6, 1917, a century ago this Thursday.

The war is best remembered for the brutal trench warfare, the millions of deaths and the failure to bring a lasting peace to Europe. But the conflict also saw a convergence of emerging technologies that would remake life on and off the battlefield.

Before the war, wristwatches were worn mostly as jewelry by upper-class women. But they become essential gear for front-line officers. The one at the museum has white numbers tipped with radium, a radioactive substance that makes them glow in the dark, and there's a metal grill that partly covers the watch face.

"This guard is often referred to a shrapnel guard, so that if a soldier is out in the field, the guard protects the tender dial of the watch," said Stephens.

These watches were wound by hand, and we found one that's still in working condition at the National Watch & Clock Museum in Columbia, Pa.

After the war, watchmakers saw a marketing opportunity and soon watches became a common sight on the wrists of men and women. It's just one small example of what grew out of the the first real technology war.

Machine guns were introduced, as were tanks, radios, military aircraft — and chemical weapons.

"The soldiers rode in on horseback and flew out on airplanes," said Libby O'Connell, a commissioner on the World War I Centennial Commission, created by Congress to mark this year's anniversary.

Like many historians, she feels Americans don't appreciate the ways the war and its technology transformed this country and the world.

The day after Congress declared war, the U.S. government banned private radio stations and equipment, fearing it might be misused.

"What the government does by taking over the radio industry, they pour funding into it for research and development, and were able to really fund technological advances that would have otherwise taken years," said O'Connell.

One result was the emergence of the first commercial radio stations in the U.S. shortly after the war.

And these advances merged in ways no one had imagined before the war.

Radio and phone communications proved vital to orchestrating troop movements along a front line that stretched hundreds of miles. Wristwatches also proved critical for pilots, serving as a backup fuel gauge.

"If you were out too long as a pilot, you didn't make it back, because you ran out of fuel," said Stephens.

Perhaps you've never made the connection between tank treads and the distinctive metal wristband on some Cartier watches. But it's straight from the war.

"That rotating wheel design is not only the way tanks will be able to cover muddy ground, it becomes the design for wristwatch bands," said O'Connell.

Louis Cartier, who founded the watch company, came up with the design after seeing France's Renault tank. On its website today, Cartier boasts that the tank watch prototype was a gift to Gen. John Pershing, commander of the U.S. forces in Europe, before it was introduced to the public.

Cartier's Tank Americaine watch is still sold today, in more than a dozen styles ranging in price up to $67,000.

Of course, warfare and technology also collided in nefarious ways that produced mass death, most notably with chemical weapons.

Christopher Capozzola, who teaches history at MIT, notes that the war came at a time of unprecedented technological innovation in the civilian world.

"Then what happens during the war is that those technologies are turned from production to destruction," he said. "Almost any of the new technologies that are used on the battlefield in World War I are things that had first actually been innovated in some other area of American business."

But when the war ended, the U.S. emerged as a true military power, with the world's largest economy, a more global mindset and a new appreciation for technology.

"World War I will change everything," said O'Connell. "It changes the world in a way that turns it into the modern world. This will be the beginning of the truly modern era."

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