Why?

In four short years, from 1914 to 1918, over 10 million men and women serving in armed forces on fronts around the world were killed, while double that number were wounded, disabled and disfigured; and at least another 7 million civilians lost their lives as well. Most died horrific deaths. But as time passes by we tend to forget, a century later, how many sacrifices were made day after day on both sides of one the most deadly conflicts in human history. Civilized Productions has produced a wonderful choral album, Sacrifice and Solace, which features an octet called the Toronto Valour Ensemble who sang these carefully selected and uniquely composed songs from that era. It is available on CD Baby. The simple translation of the Arabic word "jihad" is struggle.

The Struggle for Financing

Wars not only cost lives, they cost money. French nationalists, after losing the Franco-Prussian War and subjected to (and obligated to pay) war reparations totaling five billion gold francs, took their pound of flesh in return at the Treaty of Versailles in 1919 forcing Germany to pay 132 billion gold marks to the victors. Although suspended by Adolf Hitler prior to the outbreak of WWII, and which was reduced after the war in 1953, Germany actually paid the last of the debt in 2010.
As a sidebar, it is interesting to note after the Second World War German leaders agreed not only to a policy of pastoralization but to pay the Allies in machinery and manufacturing plants, whereby factories were actually dismantled and sent to France and Britain, while the United States harvested its technologies and patents, as well as scientists and researchers. Also, in addition to cash paid, millions of prisoners and civilians were forced to work in European and Russian plants for years.
It has been calculated that the Great War in total cost the Allied Powers, from 1914 to 1918, over 125 billion dollars, whereas it cost the Central Powers over $60 billion. In retrospect, the war directly led to the Great Depression, World War II and the Holocaust, and the Cold War, impacting the lives of many more millions of people around the world, including immigration and emigration, as the various empires collapsed and better opportunities were sought for children across oceans. 
In the United States, World War I led to prohibition, women's suffrage and eventually civil liberties. The war not only led to the collapse of the Ottoman, Austro-Hungarian, German and Russian (and some would add the British) empires, but sparked colonial revolts in the Middle East and Vietnam. In Canada, both conscription and income taxes were introduced (temporarily) during the war; in fact, a lack of income tax was provided as an incentive to attract immigrants before the war.
After the war, independent republics were formed in Austria, Czechoslovakia, Estonia, Hungary, Latvia, Lithuania, and Turkey. The latter had controlled most Arab lands which subsequently came under the control of Britain and France, specifically their oil companies. Sadly, the mass murder of Armenians in Turkey was a consequence of the war, as was the influenza epidemic that killed over 25 million people worldwide, more than the war itself.
Nonetheless, there were some positive advancements made during the war, such as those related to hospitalization, medical care and treatments in general for the wounded, sick and infected, at home and on both the Western and Eastern fronts, as well as battles fought at sea and in the air, and in African nations, Italy, Greece, the Balkans, the Middle East and the Falkland Islands. 
Quite a few innovations were to emanate from the war:  sanitary napkins, facial tissues, the sun lamp, daylight saving time, tea bags, the wrist watch, the zipper, stainless steel and vegetarian sausages.
It has been well documented that many companies benefited financially from their inventions and contributions to the war effort, on both sides of the conflict and for various reasons, balanced between patriotism and profit. As we understand it today, prior to 1914 nations simply did not have military-industrial dependencies in place to finance research, develop infrastructure and indeed manufacture and distribute the various disparate materials necessary to win a modern war. 
Gillette made and sent to the front millions of razors which were needed and eagerly used by unshaven soldiers, especially those who had suffered and died enduring a poison gas attack while fumbling to put on a rubber mask atop their (lice-filled) beards. It is no secret millions of pens made their way to the front in kits for soldiers needing in a moment of solace to put their thoughts to paper and send quiet reflections back home to friends and family members.
One such note was a poem written by a Canadian doctor during the Second Battle of Ypres in 1915, titled In Flanders Fields; it was used in advertisements by the British and Canadian governments to raise funds from the public by helping to sell war bonds at home, which ultimately made John McCrae famous, but he died of an infection on the Western Front before the war ended. These much-needed funds were to offset the millions already being borrowed from the financiers of the day.
In 1918, over 4,000 soldiers of Britain's Army Postal Service were delivering twelve and a half million letters a week to the front. During the Great War the fountain pen was not an insignificant weapon, as these letters were known to be a morale booster for their armed forces.
Stateside, a Christmas-season Parker ad shows a firm-chinned, mustached army officer and a boyish, smiling aviator wearing a leather helmet and goggles, who emerge from the leaves of a holly wreath, framing a popular wartime catch-phrase: "Keep the home fires burning," while the Sheaffer Pen Company assured customers its lever filler was the pen "for Uncle Sam's Fighting Boys" and was "The Gift of Gifts for Soldiers and Sailors" to boost sales. 
At the outset of the First World War, many countries were ill-prepared to finance their initial efforts, not to mention maintain and increase operations on a global level for the next four years. In 1913, following the passing of the Federal Reserve Act, US Congressman Charles Lindbergh stated: "The Act establishes the most gigantic trust on earth. When the President signs this Bill, the invisible government of the monetary power will be legalized. The greatest crime of the ages is perpetrated by this banking and currency bill." The Federal Reserve is in fact a private company.
In Europe, in 1914, the Rothschild family loaned money to the Germans, British and French, while also controlling three of the leading European news agencies: Wolff in Germany, Reuters in England and Havas in France through which public interest in these respective countries was manipulated. 
Mayer Amschel Rothschild founded in the late 18th century an extensive banking dynasty by sending four of his sons to major European cities with instructions to remain loyal to the family at any cost (the fifth son remained behind to manage the family business in Prussia) whereupon they bankrolled various kinds of national infrastructure projects. They were family bankers indeed, but the families with which they did business were the royal houses of Europe. 
Despite its neutrality, the US was not immune to the pressures of its financiers. "The war should be a tremendous opportunity for America," wrote J.P. Morgan in a letter to President Woodrow Wilson on September 4, 1914 immediately after the outbreak of hostilities along the Western Front. 
Twenty years later the US Senate would conduct an investigation into the causes of WWI and, specifically, why the United States had entered it. Called the Nye Committee, it was headed by Senator Gerald Nye (R-North Dakota). Their investigation focused primarily on the munitions industry and its influence on the federal government, but also on the Wall Street banks.
After nearly two years of investigation the committee found that between 1915 and January 1917, before entering the war, the United States had lent Germany 27 million dollars, while at the same time lending Britain, France and their allies $2.3 billion!
When the Russian government began to topple after the February Revolution in 1917, Wall Street pressured Wilson's government to come to the aid of their allies, and their outstanding loans. Within months the United States was at war. Due to Nye's investigation, it is known the profits that Wall Street banks made from these war loans were enormous.
Before the war had begun, the French firm of Rothschild Freres had cabled Morgan and Company in New York asking for $100 million. From that point going forward, J.P. Morgan became the main point of contact in the United States for allied war effort borrowing.  
After surveying the U.S. mobilization and financing for the war, the National Bureau of Economic Research's Hugh Rockoff stated that perhaps the greatest impact of World War I was a shift in the landscape of ideas and the proper role of government in economic activities.  When the war began, the U.S. economy was in recession. But a 44-month economic boom ensued from 1914 to 1918, as Europeans began purchasing U.S. goods and later as the United States itself joined the war. 
In 1914, the United States was a net debtor in international capital markets, but following the war began investing large amounts internationally, particularly in Latin America, thus "taking on the role traditionally played by Britain and other European capital exporters." With Britain weakened after the war, New York emerged "as London's equal if not her superior in the contest to be the world's leading financial center." 
Rockoff concluded that the scope and subsequent speed of government expansion were likely due to the emergence new economic and political leaders, who in turn inspired future generations of reformers with a new view of the world and its order.