The two main alliances were the Triple Alliance and the Triple Entente. The Triple Alliance consisted of Germany, Austria-Hungary, and Italy. The Triple Entente consisted of Britain, France, and Russia.
The assassination of Archduke Franz Ferdinand of Austria-Hungary in 1914 sparked a chain of events that led to the outbreak of war. Austria-Hungary declared war on Serbia, Russia mobilized its army in support of Serbia, and Germany declared war on Russia and France. Britain then declared war on Germany.
The system of alliances meant that each country was obliged to come to the aid of its allies if they were attacked. This made it impossible for any one country to remain neutral in the conflict, and led to the spread of the war across Europe.
In conclusion, the system of European alliances was a major factor in the outbreak of World War I. It created a situation where any conflict between two countries could quickly escalate into a wider war, and made it difficult for any country to remain neutral.