On June 28, 1914, Archduke Franz Ferdinand, heir to the Austro-Hungary throne, was assassinated by a Serbian nationalist group called the "Black Hand". This was the beginning of a series of events that lead to the First World War.
On July 28, exactly one month after Ferdinand's assassination, Austria-Hungary declared war on Serbia. Russia came to Serbia's defense, recruiting its very large army. It took a full six weeks to bring the entire army together.
On August 1, Germany, in defense of its ally, Austria-Hungary, declared war on Russia.
France retaliated and went to war on Russia's behalf against Germany and Austria-Hungary. Britain, an ally to France, declared war on Germany on August 4. Japan, as Britain's ally, declared war on Germany on August 23 and, in return, Austria-Hungary declared war on Japan. Italy managed to avoid the war until May 23, 1915.
This was just the beginning of the war. There were numerous other countries that were involved. The war involved countries in North America, South America, Asia and Europe. One country was attacked, causing her ally to retaliate. This became a huge cycle causing a world wide "domino effect."
Involving most of Europe and spanning over four continents, World War I was the first truly global war. On August 19, 1914, The United States declared a "policy of absolute neutrality." They kept this position almost through the entire war. On April 6, 1917, the United States officially entered the war after Germany's policy of unrestricted submarine warfare nearly wiped out America's commercial shipping. This policy allowed Germany to sink any ship that approached Britain regardless of whether it was a military ship, supply ship or even passenger ship.
The war ended on November 11, 1918 when Germany signed an armistice treaty with the allies.
The war produced over 40 million casualties. 20 million lives were lost with as many civilian deaths as military deaths. 10 million civilians and nearly 10 million soldiers lost their lives. There were also 21 million injuries. World War I marked the end of the world order which had existed and would be a factor in the outbreak of World War II. Ironically, the First World War became known as "the war to end all wars."