The first world war started in 1914. The world war started when Germany started acquiring weapons which spiked the world leaders to warn.
Few months later, Germany invaded Poland and other countries. To also note that Germany had alliance with Turkey and Italy to start the world war.
And on the other hand, The United Kingdom (UK) had alliances with France and other countries to stop Germany from acquire more state.
By the end of 1918, Germany and Russia surrender and have a peace treaty.