World War 1 was fought from 1914 to 1918, in which Great Britain, France, Russia, Belgium, Japan, the United States, and other allies defeated Germany, Austria-Hungary, The Ottoman Empire, Italy, and Bulgaria.
World War 1 began as a small regional conflict in the Balkans (South-East Europe). The Ottoman Empire had lost many states in this part of the world so political power in this area was very unstable. Later Austria-Hungary annexed Bosnia from the Ottoman Empire. This is where the spark that started WW1 happened.
On Bosnia’s National Day, Archduke Franz-Ferdinand visited Bosnia’s main city, Sarajevo, as a sign of Austro-Hungarian power in the region. Little did he know a group of Serbian terrorists called the Black Hand Gang were about to assassinate him in the aim of freeing Bosnia from Austro-Hungarian rule. The initial assassination attempt failed but later a fleeing Black Hand Gang member called Gavrilo Princip saw the Archduke and shot him in the neck and his wife in the stomach. This assassination ignited WW1, but as Germany had nothing to do with it how could they be solely to blame for causing WW1. However this does not mean Germany was not partially to blame for WW1.
The assassination itself was not Serbia’s doing but nevertheless Austro-Hungary sent an Ultimatum to Serbia as punishment for the assassination. It was made deliberately too harsh to comply with so that Austria-Hungary had an excuse to invade Serbia. Whilst this may seem like it was Austria-Hungary’s fault for behaving so recklessly with their demands, it was also Germany’s fault because they offered Austria-Hungary a “Blank Cheque” agreement saying that Germany would support Austro-Hungary’s Ultimatum even if it meant going to war. Therefore this allowed Austro-Hungary to behave recklessly with their demands as they had the support of Germany behind then. This means Germany is at least partially to blame for starting WW1.