Germany sought an alliance with Austria-Hungary and Russia.
Germany created the Duel Alliance, under which Germany and Austria-Hungary agreed that each would help the other if either was attacked by Russia.
In 1882 Italy joined Germany and Austria-Hungary to become the Triple Alliance.
Germany ailed to renew a treaty with Russia, France found an ally in Russia.
Franco-Russian Alliance each agreed to help the other if attacked by Germany.
Alarmed by Germany’s move (of building up the German navy, and creating a colonial empire), Britain signed the Entente Cordiale with France in 1904.
Britain and Russia settled their differences. Britain, France and Russia linked up in the Triple Entente.
There was tension between Austria and Serbia. In 1908, Austria took over two Turkish Balkan provinces where Serbs took up most of the population.
On the 28 July 1914, Austria declared war.
Serbia saw Austria as the main threat to its growth.
On the 23 July, Germany gave Austria a guarantee of military support and Austria presented Serbia with an ultimatum (a final set of demands or terms backed by a threat).
On June 28 1914, during an official visit to the Bosnian town of Sarajevo, the heir to the Austrian throne, Archduke Franz Ferdinand and his wife were fatally shot. There killer was a Bosnian-Serb.
Italy withdrew from the Triple Alliance and joined the Allies in May 1915. As the war progressed countries such as Greece, Portugal, Romania, Japan, China, Brazil joined the Allies.
Austria and German troops overran Rumania soon after they joined the Allies.
Despite some victories against Austria-Hungarian forces, Russia’s military was collapsing by early 1917 and the Revolution of November 1917 ended Russia’s involvement in the war.
The United States of America joined the Allies.