In 1914 in Sarajevo when Gavrilo Princip, a member of the Black Hand, assassinated the Archduke of Austria, Franz Ferdinand there was an outrage in Austria-Hungary to retaliate. Blaming Serbia for the death of Franz Ferdinand and his wife, Austria-Hungary got the blank check from Germany and declared war on Serbia on July 28, 1914.
After Austria-Hungary declares war on Serbia on July 28th, 1914, Russia begins to mobilize their troops two days later due to their alliance with Serbia. In response, Germany declares war on Russia and her allies this far, being France and Serbia. By August 4th, 1914, the countries at war were Russia, France, Britain, and Serbia versus Germany and Austria-Hungary.
On August 3rd, 1914, Germany invades a neutral Belgium and begins to wreak havoc because of the small amount of resistance they face from the Belgian civilians. They burn houses and murder helpless citizens. They also invade Poland, a Russian territory, on August 4th and take three of the towns. Then, in solidarity with Britain and a few anterior motives, Japan declares war on Germany on August 23rd, 1914.
On April 22nd, 1915, Germany starts using coloring shells and fires them at Allied lines. In France, it was almost enough to break the line, but the Germans could not advance due to the gas that still lingered in the air.