World War I:
World War 1 lasted from 1914 to 1918. World War 1 began as a local European conflict between Austria-Hungary and Serbia on July 28,1914. It was transformed into a general European war by the declaration of war made by Germany against Russia, on August 1,1914.
These intermingled conflicts eventually became a global war involving thirty two nations, twenty eight of which, known as the Allies and the Associated Powers, and including Great Britain, France, Russia, Italy and the United States, opposed the coalition known as the Central Powers, consisting of Germany, Austria-Hungary, Turkey and Bulgaria.
The immediate cause of the war between Austria-Hungary and Serbia was the assassination on June 28,1914, at Sarajevo, capital of the Austro-Hungarian province of Bosnia, of the Archduke Francis Ferdinand, heir-presumptive to the throne of Austria-Hungary, by Gavrilo Princip, a serb nationalist.
The fundamental causes of the conflict, however, were rooted deeply in the European history of the previous century, particularly in the political and economic policies of the nations of Europe after the year 1871, which marked the emergence of Germany as a great world power.
The unarmed British passenger ship Lusitania was torpedoed and sunk by a German submarine on May 7,1915. This outraged the American public opinion against Germany. Hostilities between the Allied and Central Powers continued until the signing of the Armistice on November 11,1918, a period of four years, three months and fourteen days. The aggregate direct war costs of all the belligerents amounted to about 186 billion dollars.
Casualties in the land forces amounted to over 37 million; in addition deaths among civilian populations caused indirectly by war approximated 10 million.
World War 2, also known as the Second World War, was a war fought from 1939 to 1945 in Europe and, during much of the 1930s and 1940s, in Asia.
The war in Europe began in earnest on September 1,1939 with the invasion of Poland by Nazi Germany, and concluded on September 2,1945, with the official surrender of the last Axis nation, Japan. However, in Asia the war began earlier with Japanese interventions in China, and in Europe, the war ended earlier with the unconditional surrender of Germany on May 8,1945.
The conflict spilled over into Africa, included a handful of incidents in the Americas, and a series of major naval battles. It was the largest armed conflict in history, spanning the entire world and involving more countries than any other war, as well as introducing powerful new weapons, culminating in the first use of nuclear weapons.
However, despite the name, not all countries of the world were involved; some through neutrality (such as the Eire - though Eire supplied some important secret information to the Allies; D-Day’s date was decided on the basis of incoming Atlantic weather information supplied from Ireland - Sweden, and Switzerland), others through strategic insignificance (Mexico).
The war ravaged civilians more severely than any previous conflict and served as a backdrop for genocidal killings by Nazi Germany as well as several other mass slaughters of civilians which, although not technically genocide, were significant. These included the massacre of millions of Chinese and Korean nationals by Japan, internal mass killings in the Soviet Union, and the bombing of civilian targets in German and Japanese cities by the Allies. In total, World War II produced about 50 million deaths, more than any other war to date.