Between April 1917 and November 1918, the U.S. was involved in the first truly global war. 

Over two million Americans fought in the biggest and most costly war in European history to that date. Entering only at the tail end of four years of slaughter and horror, the United States helped turn the tide of the war in favor of the Allies, and brought America onto the international stage as a major military, financial, and industrial power, altering the course of history.

And not just American history—the war was literally fought all over the world, and killed about thirty seven million soldiers and civilians. It led to revolutions, ended and expanded empires, and ramped up the pace of technological change.

The largest national project the country had ever undertaken, World War I led to an increasingly large army and more power for the federal government. By the war's end, over 50,000 American soldiers lay dead on Flanders' Fields, with even more felled by disease.

For the United States, it marked the end of its isolationist era. 

It was like the awkward kid playing alone in the sandbox with its Tonka trucks, refusing to join in the giant game of freeze tag everybody else was playing. But then the freeze tag game grew so massive, and so violent, and so important, that America just couldn't ignore it anymore. U.S. involvement in the war was costly, but it established America once and for all as a world power.

But wars affect a country's juicy innards, too, not just its external affairs. During WWI, the U.S. launched one of its first propaganda programs, instituted the draft, and came to see some of its own internal problems—cough, racism—slightly more clearly. 

Then it had to deal with all the messy fallout afterward. Little things like legal infringement on our First Amendment right to free speech. No biggie.

Ever wonder why we celebrate Veterans Day on November 11th? No? Okay, ever wonder where the phrase "down in the trenches" comes from? Still no? How about a trench coat, or shell shock? Ever wonder why there are three totally separate ethnic groups living in the single country of Iraq? 

Well, these are all artifacts of World War I, one of the most deadly and horrible wars the world has ever seen. Between 1914 and 1918 more than 11 million soldiers died in a war that began with the shooting of an archduke after his driver took a wrong turn in Sarajevo, Serbia. 

In many ways, the modern world began in the trenches of the Western Front, and the United States rose from a quiet industrial power to world preeminence thanks to its role in the so-called "War to End All Wars."

World War I didn't do that of course—there was a World War II, after all—but the changes in warfare that came out of the conflict are still being felt today.

When war broke out in July 1914, many people were relieved that the long-awaited showdown between Germany and her allies—the Ottoman and Austro-Hungarian Empires—against Britain, France, and Russia had finally come. The general sentiment was that the war would be over by Christmas. 

The reality was far different. After two months of old-fashioned mobile warfare involving cavalry and infantry battles, the fronts in both Eastern and Western Europe hardened into a stalemate with both sides hunkered down behind increasingly sophisticated trench systems, some of which can still be seen today lining the fields of Belgium and France. 

The carnage of trench warfare was incredible. On one single day, July 1st, 1916, the British army suffered 58,000 casualties in the Battle of the Somme, all in a futile effort to capture just a few hundred yards of territory. An entire generation of European men died in the trenches. Farmers in northeastern France today still routinely dig up bombs, bones, and other rubble from the war.

The United States stayed out of the war for almost three years before finally throwing its hat into the ring in April 1917. By the end of the war on November 11th, 1918—still known Armistice Day in Europe, but renamed Veterans Day in the United States in 1954—two million American soldiers had taken part in the fighting in Europe. Within four short months in early 1917, the United States reversed course and pursued an interventionist foreign policy. 

That is, we meddled in other people's business with a very big army.

World War I, though it has been called the "War to End All Wars," led directly to World War II and helped forge the international order we know today. Following the end of the war, the Ottoman Empire was divided into 'Protectorates' under the control of Britain and France. One of those was called Palestine; another was called Iraq. 

Many of the recent historical causes of war and strife in the Middle East can be traced back to two members of the British and French Foreign Offices—Sykes and Picot—who drew some lines on a map and came up with the modern Middle East.