Germany was always overlooked as the underdog, the country that was barbarian-like. During the 1900s, the German industry was rapidly increasing, challenging Britain to its exports, and length of transportation and army size. Then, as that happened, the Germans became proud of themselves, and wanted to overpower Britain and become the superior empire. They challenged Britain and felt as if there was no way that they could lose.
On the other hand, Britain had centuries of power and had always been superior. When Germany started threatening its power, they panicked. The Germans' attitude towards war was offensive while the British were defensive, saying that they'd only fight if they had no other choice while protecting their nation and colonies.
While all that was happening, Austria-Hungary was trying to control its colonies while Serbia and the Balkan countries wanted independence. So when Archduke Franz Ferdinand visited Serbia on their national holiday, the Serbians were offended and decided to attempt to assassinate him and his wife. Although the assassination was amateur, the Archduke and Archduchess were still assassinated after two pathetic attempts.
With that, Austria-Hungary consulted Germany on their plan to declare war on Serbia and Germany promised their support. The Germans and Austria-Hungary felt superior since Serbia was such a small and weak country that had no colonies. Little did they know that Serbia consulted Russia and the Russians promised their support if anyone would ever attack Serbia, that they would be there for them. With that, Austria-Hungary declared war on Serbia and that triggered WWI.
Following that, Germany declared war on France, and Britain declared war on Germany and etc.