In the Black Hand organization, one person who strongly supported Serbian power sent men to kill Archduke Franz Ferdinand of Austria. They all failed to kill him with grenades while he passed through a large crowd but one of these men, a Serbian student named Gavrilo Princip, shot him and his pregnant wife with a pistol.
Austria-Hungary blamed Serbia for the assassination. Germany supported Austria-Hungary and promised full support should it come to war. Austria-Hungary sent a July Ultimatum to Serbia, listing 10 very strict conditions, possibly because they wanted an excuse to start the war. Serbia agreed to most of the ten things on the list, but not all of them. Austria-Hungary then declared war on Serbia. This quickly led to a full-scale war. Both countries' allies became involved in the war in a matter of days.
Russia joined the war on Serbia's side because the people of Serbia were Slavic, like Russia, and the Slavic countries had agreed to help each other if they were attacked. Since Russia is a large country it had to move soldiers closer to the war, but Germany feared that Russia's soldiers would also attack Germany. Russia did not like Germany because of things Germany had done in the past to become stronger. Germany declared war on Russia, and began to carry out a plan created long before to fight a war in Europe. Because Germany is in the middle of Europe, Germany could not attack to the east towards Russia without weakening itself in the west, towards France. Germany's plan involved quickly defeating France in the west before Russia was ready to fight, and then moving her armies to the east to face Russia. Germany could not quickly invade France directly, because France had put a lot of forts on the border, so Germany invaded the neighboring country of Belgium to then invade France through the undefended French/Belgian border. Great Britain then joined the war because Great Britain had agreed to help Belgium if it were ever attacked by someone else.
Soon most of Europe became involved.