To counter the growing power of Germany, France and the Russian Empire in 1894 signed an Alliance of mutual protection. Thus if Germany attacked in the West it would be vulnerable in the East, and Vice Versa. This forced Germany to Ally more closely with her neighbour, and partially Germanic Power, Austro-Hungary.
The Entente Cordiale (meaning friendly relations, or community of interests) signed in April 4 1904 between the British and French Empires, greatly improved relations between them. It fixed Imperial Frontiers, and reduced Hostility, that had existed since the English controlled Normandy, through the French lose of Quebec and support for the Americans in their War of Independence, and of course most of all the Napoleonic Wars. It was caused in large part by the growing power of Germany. Britain was particularly worried about the growth of the German Navy, as the British Empire was largely controlled by Sea Power, and Britainís only real defence against Continental Armies was the Sea. Britain had only one real interest in Continental Europe, and that was to maintain the balance of power. In 1907 an Anglo-Russian Alliance was added thus creating the Triple Entente.
The Ottoman Empire Naturally Allied with Germany, as Britain had seized its territory in Egypt, France in Algeria, and Italy in Libya. Russia had also attempted to invade Turkey during the Crimean War, in the hope of gaining access to the Mediterranean Sea, and a 'Warm Water Port'. Germany, and Austro-Hungry also had large trade interests with the Ottoman Empire, and all were hostile to the Russians.
In some ways the War could have been said to have been caused by Industrialization and improved Communication. Industrialization allowed vast amounts of Arms to be stockpiled. This lead to a giant arms race where weaker powers could increase their strength by concentrating on Military Research and Production, instead of creating things to improve Citizens quality of live, this is what H.G.Wells called the 'paradox of over production'. Thus when the War did come no one could win without totally destroying the enemy. No War before had had casualties that measured in the Millions.
The German Strategy was simple, knock France out of the War, and then take care of Russia. Unfortunately France had the Franco-German Boarder heavily Fortified, so the German High Command decided to go through unfortified Neutral Belgium. Britain had a treaty guaranteeing Belgium Neutrality, in case of such an eventuality, which forced Britain to join the War. Of course Britain didn't believe Germany would be stupid enough to go to War with the World Greatest Power, at that time Britain had the largest Fleet, and controlled one quarter of the Worlds Land Surface.
The German Gamble failed, largely due to British Reinforcements, and a War of Attrition began on the Western Front, with vast Fortifications stretching from the Channel to the Alps. Heavy casualties occurred, largely caused by the 'big push' mentality, where a rational policy of defence, was replaced by an irrational policy of attack hoping given enough force the Enemies line would break. This policy was caused by Imperial Pride, and democracies being forced to try and 'end the war quickly'. Unfortunately, the weapons available at the start were much better at defence than offence, and any point attacked could be reinforced quickly. This caused French soldiers to strike, and refuse to attack, only defend. There was no strategy for this kind of Total War, Cavalry being of no use what so ever, even though a large amount of it was maintained behind the lines.
The dead lock was eventually broken by the development of new tactics and weapons. Including Fighters (also used for Reconnaissance), Bombers, Chemical Warfare, Close Artillery Support, and 'General' Tank (useful but not critical). Prop gander and Moral also played a part, the Tanks abilities were over rated to demoralise the Germans, this lead post-War Germany to concentrate its research in this area, and create much more effective Tanks.
Japan being allied to Britain joined the War on August 1914. But the main effect of this was Diplomatic rather than military. Turkey joined the Central Powers on November 1914. In retrospect it seems a big mistake, she had little to gain and much to lose. The British making no progress on the Western Front turned their attention to the Ottoman Empire. Britainís vast Fleet allowed her to project a considerable amount of power in this area, and she controlled Egypt, next to Ottoman Territory. This boarder was however well protected, by land and sea, so Britain landed British and Australian Troops near Constantinople (Istanbul) at Gallipoli in the Dardanelles to try and knock Turkey out of the War. The landing was well protected geographically and militarily, and so Britain was forced to withdraw with large loses. This helps to account for the fact that 1 in 10 Australian Men died in World War I.
Britain was thus forced to turn her attention to the Egyptian Boarder. This area was populated by Arabs hostile to their Turkish masters, who had kept them in subjugation for Centuries. The insult being greater for once the Arabs had controlled an Empire stretching from the Atlantic to the Indus. A British Military Officer by the name of T.E.Lawrence (Lawrence of Arabia), helped unite the warlike tribes of Arabia, teach them modern military strategy, and supply them with good weapons. This rebellion in the rear combined with a modern army in Egypt, lost Turkey a vast area of territory stretching from Arabia to todayís Turkish Boarder. Mesopotamia (Iraq) was captured by troops from British India. Turkey had the good sense to stay out of the Second World War, and get some decent Allies during the Cold War.
It should be noted that during this time the Turks exterminated about 5 millions Christian Armenians living in their territory, who had been living their long before the Turks migrated there from Central Asia in the 1300's. It is probable Russia if she had wanted could have prevented this tragedy. It should also be noted that Hitler got his idea for the 'final solution' from this incident, noting what little fuss was made of it, he thought no one would care if a similar thing happened in Germany, how wrong he was.
Italy joined the War on May 1915, being promised the Catholic territories of Slovenia, Croatia, and Bosnia (also Muslim/Orthodox). This promise was never carried out, in large part due to American opposition, and was a contributory factor in Italy siding with Germany in the Second World War. It also meant the Balkans were a lot easier for the Nazis to subjugate than they otherwise might have been. The Italians did however gain a small section of Austrian Territory which they retained by first Allying with Germany, and then joining the Allies in the Second World War.
War on the Eastern Front achieved little either, due to poor communications. German financial backing for the Communists, who promised an end to the War, eventually caused a Revolution in March 1917, forcing Russia out of the War, at the cost of surrendering large amounts of territory. This did not bring Russia Peace however as a long Civil War followed, which in many ways was even worse, particularly for civilians.
It did Germany little good either as the Americans entered the War on April 1917, caused by accidental attacks on its shipping by U-Boats, worry about the level of casualties in Europe, possible domination of Europe by Germany, and Germanys attempts to get Mexico to declare War on America. They spent a long time training and equipping their forces, before actively fighting, and so only fought one major battle. Their main function was to robe the Germans of any hope of Victory.
Civil War continued in Russia, with foreign powers occupying many of her ports. Turkey occupied Baku, and tried to take territory in Central Asia, where Turkish speaking people live. This was all in support of the 'whites' and didn't help relations with the 'reds' when they finally won.
In January 1919 the United States, Great Britain, France, Italy, and the German Republic meet at Versailles south of Paris, once a Royal Residence. Here a peace treaty was drafted which created many new countries, and heavily punishing the central powers.
At the end of the War only four Powers had fought on the Central Powers Side (the Fourth being Bulgaria), the Allies numbered 28; many took no active part in the War. Austro-Hungary was divided between 7 countries, ripe for domination by neighbouring powers a few years later. Germany lost Territory to France, Denmark, Belgium, and Poland (the later even when the people voted to stay a part of Germany). Her Colonial Empire had also been taken by force, including a campaign by South Africa against German South West Africa, and one from Kenya against Tanzania - the Naval Blockade prevented any reinforcements getting through. It also had to pay a crippling War debt which helped create the Great Depression.
Woodrow Wilson, the American President, tried to make a lasting Peace by creating the League of Nations, which later collapsed due to German and Japanese Aggression. Also he wanted fairer boarders, and a smaller War debt for Germany. But France particularly stated that her suffering had been so great, that Germany must pay, and demanded retribution. A naval blockade on food was maintained until Germany and Austro-Hungary agreed to the terms.
The Arabs were promised independence for their help. All they got was Arabia. Mesopotamia, and the historic area of Greater Syria, became British and French mandates, France had a claim as she was Britainís Ally (Russia having withdrawn from the War), and having historic interests from the time of the Crusades. So Syria was divided into [Lesser] Syria, and [Christian/Muslim] Lebanon for the French, and [Jewish Homeland/Christian Arab/Muslim] Palestine, [Trans] Jordan, and Iraq for the British. These states were promised independence sometime in the future. Perhaps if Syria had been left intact the recent history of the middle east might of been less bloody, after all Jews and Arabs share blood. The Jews and Christians could never have become a majority in greater Syria, thus reducing tensions and potential for extremism and emigration. Also the Soviets would have had less interest in the area.
The final effect of the Great War was to reduce European Power, much of which moved to America, due to British and French Capital being transferred for War Arms. It also changed Europe socially, Woman became more free (due to being drafted into the work force), the Birth Rate fell, and non-Europeans realised they might not have to live under European domination forever.