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 Early Victories

Planning for a war in the early part of the Twentieth Century, leaders of the German army had hoped to sweep, uncontested, through Belgium and into France, seeking merely the quick capitulation of an their enemy who they had recently fought and beaten in the Franco-Prussian War, acquiring the territory of Alsace-Lorraine as part of their settlement, which was then re-acquired by the French after the Great War at the Treaty of Versailles. However, the Franco-Prussian War had lasted only several months, from July 15, 1870 to February 1, 1871.
During the Napoleonic wars Napoleon Bonaparte and his army had smashed through the German states with ease on their way to Russia. A generation later, roles would be reversed and the Franco-Prussian War changed European history. The rapid and overwhelming victory of the German states under the leadership of Prussia in this 19th Century conflict directly led to the creation of a unified Germany and brought down Napoleon III's empire, which was replaced by the Third Republic. Exiled, Napoleon III died in London in 1873.
Their impressive land forces ensured Imperial Germany was recognized and appreciated across Europe as a dominant military power and, aided by a modernized Navy, their guns assured their role as a global superpower as their acquisitions mounted. Prior to the ascension of Kaiser Wilhelm II, Germany had been divided primarily by religion, with the Protestants in the North and the Catholics in the South. Under Otto Von Bismarck, and his quick and successful wars, Germans were united in their hatred of France, who had conscripted 250,000 men into Napoleon's armies.
Under Bismarck's leadership, the German army trained and maintained formidable reserves, which led to victories against both Denmark and Austria, as well as subsequently France. Since its defeat by Wellington's combined armies at Waterloo in 1815, France had become a subordinate power in Europe. Napoleon III, hoping to defeat Prussia, sought to recover the Rhine frontier lost after the defeat of Bonaparte. Many French military leaders were shocked by the Prussian defeat of the Austrians an Koniggratz in 1866 and urged immediate military reforms.
Yet in 1869 a pretext for war presented itself when the Spanish parliament offered the throne of Spain to Prince Leopold of Hohenzollern-Sigmaringen, nephew of King Wilhelm I of Prussia. The throne was vacant after a revolution in 1868 had deposed the Bourbons. If he could place Prince Leopold on the Spanish throne, Bismarck hoped to provoke war with France. Upon Bismarck's insistence, Leopold accepted the offer and on July 2, 1870, the Spanish informed the French ambassador of their choice. 
Two weeks later, rejecting Britain's offer to mediate, the French Chamber declared war on Prussia, as expected. The German states, seeing France as the aggressor, came to Prussia's support. The Germans, with a national army organized under universal military service, and an efficient use of new railroads, mobilized their forces, who quickly proved their superiority and won a decisive battle at Sedan in September, where Napoleon III was captured.
Following a three-month siege, Paris surrendered in January, and the Treaty of Frankfurt was signed on May 10, 1871, ending their war, crowning their aspirations. Previously, on Jan 18, at Versailles, the new German military headquarters, Wilhelm I was proclaimed Kaiser. France not only ceded Alsace and Lorraine to Germany but also agreed a German army could occupy northern France until an indemnity of five billion Francs was paid.
Under the new constitution the southern German states were annexed by the North German Federation. There were 25 states in the new unified Reich: 4 kingdoms (Prussia, Bavaria, Saxony and Württemberg), 6 grand duchies, 5 duchies, 7 principalities and 3 free cities, certainly upsetting the delicate balance of power that had been created with the Congress of Vienna in the wake of the Napoleonic Wars. 

French anger at the Germans over the loss of Alsace and Lorraine, not to mention the large indemnity, would lead to a permanent state of crises between the two states and trigger events which would launch World War II . The First World War also influenced Italian history and its struggle for unification. With the outbreak of war, Napoleon withdrew his garrison from Rome. With this garrison gone, the Italian national army was able to overtake the Papal State of Rome in 1870.
In France, growing discontent after the Franco-Prussian War led to the establishment of the Paris Commune from March 18 to May 28, 1871, which the National Assembly's army crushed and proceeded with summary executions that numbered 20,000 in one week.
However, by 1914 the French Army had 47 divisions (777,000 French and 46,000 colonial troops), with attached cavalry and field-artillery units. Most these troops were deployed inside France with the bulk stationed along the eastern frontier as part of Plan 17. A further 2.9 million men were mobilized during the summer of 1914, and heavy losses during the first months of the war forced the French government to conscript men up to the age of 45.
The development of the railroad networks in Western Europe, allowed each army to be deployed, reinforced and supplied with unprecedented speed, yet neither to gain a territorial advantage. Once deposited on the battlefield, armies could maneuver no faster than those of the Roman Empire. Although their 20th Century weapons delivered unprecedented casualties, men were trained to fight a 19th Century battle, which led to "digging in" and the arrival of trench warfare.
By the end of 1918, a total of 8,317,000 men, including 475,000 colonial troops, had been called up to fight in the French Army. France suffered 4.2 million casualties, including 1.3 million dead in the Great War. The deaths of their soldiers created 700,000 widows and more than 1,000,000 orphans. At the Battle of Charleroi on August 22, 1914, 27,000 French soldiers were killed alone. In fact 1914 was the bloodiest year for the French Army suffering an average of 2,200 deaths per day. 
The Battle and Siege of Liège, which lasted two weeks from August 4 to the 16th,) was the first battle action on the Western Front, fought between the German Imperial Army and the Belgian Army. The Belgian city, located on high ground on the banks of the River Meuse, was surrounded by fortresses. Twelve main forts encircled the city, built as defenses to protect an important route into Belgium between the Dutch border and the Ardennes forests. 
Six brigades from the German Second Army were sent to Liège to capture the forts on August 4. One German brigade succeeded in breaking through the line of forts. The Germans occupied the city on 7th August after attacks by a Zeppelin airship and artillery fire. From August 12th - 16th shells from 11 huge howitzers, these being two German "Dicke Bertha" (Big Bertha") guns made by Krupp and nine Austrian "Schlanke Emma" (Skinny Emma") guns made by Skoda, smashed the forts. 
Following the capitulation of the city, German Imperial troops marched to the equally fortified city of Namur. Meanwhile, on August 7 the French had crossed the border into German-occupied Alsace, attempting to capture Mulhouse and liberate the province from German occupation. At the Battle of Mulhouse, which lasted until the 25th of August, this important industrial city on the Rhine river was entered and occupied twice by the French, but both times the German Seventh Army retook it.
The Battles of the Frontiers (August 14th - 25th) took place along the French-German border in Alsace-Lorraine and the French-Belgian border. As the seven Imperial German Armies advanced westwards, according to a carefully timetabled, meticulously programmed plan for their invasion of France, they came up against proud and defiant Belgian and French troops intent on defending every inch of their national soil; commanded to do so. 
The situation in the Belgian area of the Sambre-Meuse rivers became critical following the capture of Liège, as the German Second and Third Armies pushed along the Meuse. Namur lay at the junction of the Sambre and the Meuse rivers, but it too could not hold out against the destructive might of the huge German and Austrian howitzers. With support from only one regiment of French troops being able to reach the city, the defending Belgian forces were compelled to leave and by August 25th Namur was occupied by German troops.
The Battle of Mons was one of the major battles in the Battles of the Frontiers and was the first encounter between British and German forces on the Western Front. The British Expeditionary Force comprised four infantry divisions and one cavalry division of the British First Army. The British had advanced through northern France and Belgium to support the French Fifth Army's left flank on the Sambre river. Having reached the area of Mons on August 22nd they encountered German patrols at Soignies, which were advancing ahead of the German First Army.
The next day, commanded by General von Kluck, the German First Army launched an attack. The British managed to hold the Germans, but realizing their smaller British force was up against a much greater force in terms of German manpower and artillery, leaders ordered a retreat. 
By the end of August the French and the German Armies had sustained some 300,000 casualties, including wounded or killed, on both sides. The German advance had successfully penetrated the French border in several places and was pressing its advance, following the withdrawing French and British forces.
Advancing towards Paris, the German Armies stretched along a line from Verdun to Amiens. By the end of August 1914, the German First Army was within 30 miles of the French capital. In early September the British had crossed the Marne river in a retreat to the south and held a position east of Paris between the French Sixth and French Fifth Armies. However, the commander of the German First Army made a fateful change to the original directive of The Schlieffen Plan, making an assumption that the Allies were not in a position to hold out against an attack on Paris from the east. 
The original Schlieffen Plan directive had been for German forces to attack Paris from the north in an encircling manoeuvre. Nonetheless, launching an attack east of Paris, the German First Army made progress in a southerly direction. This change to the plan exposed the right flank of the German attacking force, so the French and British armies carried out counter-attacks from Compiègne east of Paris to Verdun. 
On September 9, the war a month old, the German First Army pulled back as the British First Army moved in on its left flank. All the German forces in the Marne river region retreated in a northerly direction, crossing the Aisne to the high ground of the Chemin des Dames ridge. The First Battle of the Marne was a victory for the Allied Forces, and marked a decisive turn of events in the war. Germany's Schlieffen Plan was stopped in its tracks. Stalemate. 
This bitter deadlock lasted four years.