Europe as a Powder-Keg
The fundamental causes of the World War I, or the Great War as it was known at the time, were Imperialism, Militarism, Alliance System, and Nationalism.

At the turn of the century, war was imminent.  As nationalism weakened the Ottoman Empire, the Slavic peoples of the Balkan region struggled to free themselves from Ottoman control.  Serbia declared independence in 1878, allying themselves to Russia.  Serbia then wanted to unite with Bosnia and Herzegovina, which were controlled by Austria-Hungary.  Serbia could count on Russia's support, and Austria-Hungary could count on Germany's.  Because of the high tensions in this area, the Balkans became known as the "Powder Keg of Europe."   All that was needed to ignite the world into war was a spark.

The Spark
In 1914, Austrian Archduke Franz Ferdinand and his wife were visiting Sarajevo, the capital of Bosnia and Herzegovina.  Gavrilo Princip, of the Serbian nationalist group called the Black Hand, assassinated the Archduke and his wife.  Austria-Hungary made threats to Serbia, whom they blamed for the assassination.  Russia supported Serbia, and readied their military for war.  Germany declared war on Russia, and on France, Russia's ally. Germany then invaded Belgium as a prelude to invading France.  This resulted in Great Britain declaring war on Germany, which was the start of World War I.

Aspects of World War I
Most of World War I was a stalemate.  Entrenched positions on both sides were only able to move a few miles in a battle that lasted the better part of a year.  The conditions of trench warfare were brutal; trench-foot, disease, lice, and inadequate supplies made fighting difficult.  Advances in technology also played a major role.  The machine gun, the tank, the airplane, the submarine, and the use of poison gas drastically changed the face of modern war, and resulted in large numbers of casualties.

The Aftermath and its Impact on Subsequent Events
Overwhelming odds and weariness forced an end to the fighting with the defeat of the Central Powers.  The peace conference that followed was headed by the  "Big Four,"  David Lloyd George of Britain, Georges Clemenceau of France, Vittorio Orlando of Italy, and Woodrow Wilson of the United States.  Together, they drafted the Treaty of Versailles, signed in 1919, which officially ended the war.  Wilson tried to institute his 14 Points but was largely rejected.  However, the League of Nations was created as a result and attempted to act as the peacekeeper of Europe.  Unfortunately, it was weak and ineffectual due to the absence of the United States. 

The treaty, however, was more about revenge, than it was about forging a lasting peace.  Germany was forced to accept total responsibility for the start of the war.  They also were forced to pay huge reparations, and give over vast amounts of territory.  The start of World War II is a direct result of the harsh treatment of Germany after World War I. 

World War II (1939-1945)
The rise of the fascist Adolf Hitler and the formation of German totalitarianism is directly related to the Versailles Treaty.  Hitler blamed the treaty for Germany's economic trouble and made a call for German patriotism.  The Nazi Party quickly came to power in the early 1930's and used terror to achieve its goal of a strong German nation.  The most obvious example of this is anti-Semitism, the blaming of the Jewish people for many German problems.  Benito Mussolini, the fascist leader of Italy, had a similar rise and utilized the same policies.  Without the negative influence of the Versailles Treaty, Germans and Italians may not have supported the oppressive regimes of Hitler and Mussolini. 

Causes of World War II
World War II was fought for many of the same reasons as the First World War.  The Axis Powers, Germany, Italy, and Japan, were hungry for territory and resources.  Institutions like the League of  Nations, and peace treaties such as the Kellogg-Briand Pact, were weak and ineffectual.  The immediate cause of WWII was the policy of appeasement adopted by the Allied Powers of Europe, and eventually the United States.  Germany, as it had previous to WWI, was using nationalism and militarism to imperialize surrounding areas.  Austria and portions of Czechoslovakia were taken by Germany in direct violation of the dictations of the Versailles Treaty.  Likewise, Italy had taken Ethiopia, and Japan had imperialized much of China. 

Under the leadership of Neville Chamberlain, British Prime Minister, a conference was called in Munich in 1938.  Appeasement became the official policy when Hitler was allowed to keep what was already taken if he promised to stop taking territory from that point forward.  The futile hopes of appeasement were quickly dashed when Hitler annexed the remainder of Czechoslovakia.  The worst fears were realized when Hitler invaded Poland in 1939 using the blitzkrieg tactic.  Blitzkrieg, or Lightening Warfare  used all available military resources to attack the target simultaneously.  The invasion of Poland marks the beginning of World War II.

Aspects of World War II
WWII is divided into two areas, the European Theater and the Pacific Theater. In Europe, Germany had quickly taken over large amounts of territory.  France fell after French and British troops were pushed off of the European mainland at Dunkirk.  Then, Germany failed to win the Battle of Britain in the skies over England.  Italian and German troops gained control of the Mediterranean and the North coast of Africa. 

Germany also attacked the Soviet Union, despite the secret agreement between Hitler and Josef Stalin, which stated neither would attack the other.  This proved Germany's undoing as it had in World War I, as a two-front war is nearly impossible to win.  The Battle of Stalingrad was the turning point of the war in Europe.  After this Soviet victory, Allied troops retook Africa and then move on to take the Italian peninsula.  Mussolini's Italy was considered the weak point of Europe.  After the daring D-Day invasion of Normandy, and Germany's last-ditch effort in the Battle of the Bulge, it was only a matter of time before Berlin fell. 

Hitler committed suicide as Soviet troops took the city in the spring of 1945.  The surviving Nazis that were captured were held accountable for their war crimes in the Nuremburg Trials.  The atrocities of the Holocaust would come to light, resulting in the execution of Nazi Party officials and ending the darkest segment of human history.