World War 1 began on July 28, 1914 and lasted until November 11, 1918. Differences in foreign policies were to blame, although the immediate cause was the assassination of Austria’s Archduke Ferdinand.
The two main sides were the Allies, which included France, Great Britain and Russia; and Germany and Austria-Hungary. In total, 30 countries were involved in the conflict. Italy, once part of the Triple Alliance with Germany and Austria-Hungary, fought on the side of the Allies.
Soldiers fought largely in trenches during the war, and thousands suffered from stress, known as shell-shock. The British and French trenches were often squalid, whereas the German trenches were almost luxurious in comparison, with bunks and decent cooking facilities.
By the end of WW1, over 13 million soldiers had been killed, and another 21 million wounded. Over a million soldiers were killed in the infamous Battle of the Somme alone, including about 30,000 in just one day.
Around 11 percent of the population of France was killed or wounded during the war. About 116,000 Americans were killed, even though the US was only in the war for about 7 months.