Story highlights Ruth Ben-Ghiat: 100 years ago this summer, World War I began. Its legacies still resonate

She says its carnage was unprecedented; it changed military, sparked innovations

She says empires fell, borders shifted, new words emerged, and women's roles changed

Ben-Ghiat: Begun after an assassination, it would draw in 30 nations, upending the world

World War I began a hundred years ago this summer, but for many of us it might as well be a thousand. We know it, if we know it at all, as a dimly remembered chapter in high school history, or as scenes from old black-and-white movies of soldiers hunkered in trenches doing battle with Germans in pointy helmets. It was all too real for more than 65 million men from some 30 nations who were plunged into carnage the likes of which the world had never before seen.

Every one of those soldiers is dead, and the causes they fought for are lost on many of us. Yet this "war to end all wars" is not a remote event. In fact, World War I changed the world forever, and its effects are all around us.

Ruth Ben-Ghiat

To begin with, it rewrote history at the grandest level: Empires fell, and new nations--Austria, Hungary, Yugoslavia and Poland among them-- were born in the ashes. Leaders of the still-powerful French and British empires used the conflict to redraw borders in ways that set the stage for future conflicts that stretch on today, in the Middle East, for example.

But there is much more. The first mass conflict among industrialized nations, World War I upended the way war was fought. The weapons it introduced -- submarines, machine guns, poison gas, grenades, tanks -- are all still part of our arsenals. And it was World War I that made airpower and strategic bombing central to the success of any future war. Trench warfare traumatized both soldiers and landscapes, and informed art and literature for years. It would reappear as a battlefield strategy in both the Korean War and in the Iran-Iraq War of the 1980s.

At home and on the battlefield, World War I put new objects and words into circulation: "cooties" are something no kid wants to get, but for GIs in the trenches, they were real and they were lice; and sanitary napkins developed from the handy alternative use nurses found for cellulose bandage material produced for the war. The war popularized Kleenex and tea bags and zippers.

In fact, every time you admire an aircraft carrier, eat a meatless sausage, sit under a sun lamp, wear a Burberry trench coat, or set your clock ahead for daylight saving time, you are reaching back to commune with World War I.

The dawn of chemical weapons

World War I's new weapons caused previously unseen and horrific kinds of injuries, and scientists raced to develop protections against them -- or to make even more lethal versions to use against the enemy. Poison gas was first used on a mass scale by the Germans in April 1915 during the second battle of Ypres, and cloths strapped over the mouth and nose were at first the only protection.

Photos: World War I: A time of upheaval Photos: World War I: A time of upheaval French soldiers sing the national anthem at the beginning of World War I in August 1914. This "war to end all wars" might seem like ancient history, but it changed the world forever. It transformed the way war was fought, upended cultures and home life and stimulated innovations that affect us today. With more than 30 combatant nations and nearly 70 million men mobilized, World War I profoundly destabilized the international order. Look back at some of the war's key events. Hide Caption 1 of 25 Photos: World War I: A time of upheaval The bodies of Austria-Hungary Archduke Franz Ferdinand and his wife, Sophie, Duchess of Hohenberg, are seen after their assassination by Serbian nationalist Gavrilo Princip on June 28, 1914. The assassination led Austria-Hungary to declare war on Serbia, starting a chain of events that would gradually bring other nations into the fray. Hide Caption 2 of 25 Photos: World War I: A time of upheaval Germans in Berlin mobilize for war on August 1, 1914. Germany was a strong ally of Austria-Hungary. Hide Caption 3 of 25 Photos: World War I: A time of upheaval French soldiers are seen at a front-line trench in Italy. During World War I, the Allied Powers consisted of Belgium, France, Great Britain, Greece, Italy, Montenegro, Portugal, Romania, Russia, Serbia and the United States. The Central Powers consisted of Austria-Hungary, Bulgaria, Germany, and Ottoman Empire (now Turkey). Hide Caption 4 of 25 Photos: World War I: A time of upheaval German soldiers captured by the British are seen in France. Hide Caption 5 of 25 Photos: World War I: A time of upheaval Men visit the graves of French soldiers killed in the Battle of the Marne in September 1914. Hide Caption 6 of 25 Photos: World War I: A time of upheaval Troops land at Anzac Cove in the Dardanelles during the battle between Allied forces and Turkish forces at the Gallipoli Peninsula in February 1915. The two sides were fighting for access to the strategic Sea of Marmara and eventually to Constantinople (Istanbul). Hide Caption 7 of 25 Photos: World War I: A time of upheaval German and British troops are seen together during the Christmas Truce of 1914. Hide Caption 8 of 25 Photos: World War I: A time of upheaval These French Zouave infantrymen were killed by gas during the Second Battle of Ypres, Belgium, in April 1915. Hide Caption 9 of 25 Photos: World War I: A time of upheaval Germans give oxygen to a gas victim in 1915. The cloth masks worn by these soldiers provided little defense; more substantial gas masks were not produced until 1916. By the end of the war, even horses and dogs used at the front had their own gas masks. Hide Caption 10 of 25 Photos: World War I: A time of upheaval German U-boats, or submarines, patrol the Mediterranean coast. Hide Caption 11 of 25 Photos: World War I: A time of upheaval French troops rest in Verdun, France, in 1916. Verdun was the site of the longest battle of World War I. Hide Caption 12 of 25 Photos: World War I: A time of upheaval Dead bodies lie piled in a trench at Verdun on April 9, 1916. Hide Caption 13 of 25 Photos: World War I: A time of upheaval British troops advance during the Battle of the Somme in 1916. Hide Caption 14 of 25 Photos: World War I: A time of upheaval A British tank is stranded while crossing a trench during the Battle of the Somme on September 25, 1918. Hide Caption 15 of 25 Photos: World War I: A time of upheaval German soldiers stand near a crashed fighter plane in Germany in 1916. Hide Caption 16 of 25 Photos: World War I: A time of upheaval Czar Nicholas II of Russia, right, reviews the palace guard just prior to the Russian Revolution of 1917. Hide Caption 17 of 25 Photos: World War I: A time of upheaval U.S. President Woodrow Wilson addresses a joint session of Congress in April 1917, requesting a declaration of war on Germany. The United States declared war against Germany after the interception and publication of the Zimmermann Telegram and the sinking of three U.S. merchant ships by German U-boats. Hide Caption 18 of 25 Photos: World War I: A time of upheaval Britons stand in line outside an Army recruiting station. Hide Caption 19 of 25 Photos: World War I: A time of upheaval A British sergeant major instructs American soldiers in bayonet fighting at Texas' Camp Dick. Hide Caption 20 of 25 Photos: World War I: A time of upheaval U.S. Army troops stand in a defensive trench in France. By war's end, thousands of miles of trenches crisscrossed European battlefields. Hide Caption 21 of 25 Photos: World War I: A time of upheaval A defeated German army marches home on October 1, 1918. Hide Caption 22 of 25 Photos: World War I: A time of upheaval Armistice Day is celebrated in Chicago on November 1, 1918. Hide Caption 23 of 25 Photos: World War I: A time of upheaval U.S. troops returning home from France are seen on the USS Agamemnon in Hoboken, New Jersey, in 1919. Hide Caption 24 of 25 Photos: World War I: A time of upheaval German delegates listen to French Prime Minister Georges Clemenceau's speech during the signing of the Treaty of Versailles in France on June 28, 1919. Hide Caption 25 of 25

Gas masks evolved quickly, though, and by the end of the war even some horses and dogs at the front had their own. The horrors of gas attacks resonate today in the reports of chemical weapons use in Syria, and, earlier, in Saddam Hussein's Iraq, and the world still struggles to contain them.

All told, more than 9 million died in the conflict, and 21 million were wounded, psychologically scarring a generation. Soldiers were at pains to explain this new human experience of battle to those back home.

The English poet Siegfried Sassoon had this to say in 1917 about his time at the front: "I'm back from hell/With loathsome thoughts to sell/Secrets of death to tell;/ And horrors from the abyss." Many others had no more words: these victims of "war exhaustion," (the label of shell shock became more common) had trouble speaking: they are the forefathers of veterans with post-traumatic stress disorder today.

Likewise the scale and type of physical injuries challenged the ingenuity of prosthesis designers, whose work to replace lost body parts would enable countless soldiers to return to productive civilian life, a process echoed today as soldiers from recent wars recover from the toll of roadside bombs.

World War I also set the stage for future conflicts, by breaking down barriers between military and civilian life. While soldiers fought at the battlefront, civilian women and men built their weapons, distributed food and propaganda, and kept the home front running. Women gained new visibility in society, moving into the jobs vacated by enlisted men.

They drove streetcars, smelted iron, built bombs and then, after a long day at the factory, scrounged for food for their families. Civilians working for the war effort meant that anyone could be a target: German Fokker planes attacked at the front, but Zeppelin airships bombed London and Paris. "Total war" made the home front a dangerous place.

This war left few things unchanged in its path, even in lands that saw no fighting. Although it was mainly fought in Europe, it awakened many to the scope and diversity of the planet. "The entire world is participating in the war!" a French almanac exclaimed in 1917, showing its readers a map of the world divided into enemy, ally, and neutral peoples. Whether as laborers or soldiers, Europeans went to other countries, and millions of Americans, Africans and Asians came to Europe.

'Trapped in a net of woe'

More than two million United States soldiers fought in Europe, and the British and French empires brought over their colonial subjects. "We perish in the desert; you wash yourself and lie in bed," wrote an Indian soldier to his wife in September 1915. "We are trapped in a net of woe; while you go free. Our life is a living death."

How did Europe arrive at this state of catastrophe? The assassination of Austrian-Hungarian Archduke Franz Ferdinand and his wife, Sophie, on June 28, 1914 by the Bosnian Serb Gavrilo Princip caused an international crisis that led in just over a month to multiple mobilizations.

The Archduke, traveling in an open car, was in Sarajevo to inspect imperial armed forces in Bosnia and Herzegovina, which were among the former Ottoman territories annexed by Austria-Hungary in 1908, angering Serbian nationalists such as Princip.

After the assassination, Austria-Hungary gave Serbia an ultimatum, causing Russia to intervene to protect its Serbian client state, and Germany to help its Austrian ally. And so it all began: the military obligations imposed by the system of alliances drew one power after another into combat.

All parties thought the war would be a short one; none imagined the speed with which the conflict would degenerate into a series of local atrocities (the Belgians became the conflict's first group of refugees, as they fled German rape and plunder) and mass slaughter across many fronts.

The habituation to violence and the acceptance of these lethal new inventions is one of World War I's most unfortunate legacies. Chemical weapons provides a case in point. Their effectiveness, as proved by the precedent of World War I, has given them a permanent place in many state arsenals, despite the paper trail of international agreements meant to ban their use. Democracies and dictatorships (France and Italy) both used them in the interwar period as agents of colonial conquest and rule, and Syria is the most recent example of their use.

As we approach this 100-year anniversary, each combatant country is remembering the war in its own way. In America, the echo has been fainter, due as much to the country's late entry into the war (April 1917) as to the prominence of World War II.

"The First World War is not well understood or remembered in the United States, U.S. Defense Secretary Robert Gates said at a 2008 Veterans Day ceremony at which the last living American combatant, Frank Buckles, who died in 2011, was present. "Yet few events have so markedly shaped the world we live in."

At war's end in 1918, America emerged from its 18 months of combat with a raft of new legislation that is still in force -- such as the Selective Service Act, which still today allows the President to draft soldiers, and the Espionage Act, used recently to charge Chelsea Manning and Edward Snowden -- and with a new status as an international power.

A century of debates over how and whether America should intervene in global crises would lie ahead.