How did alliances contribute to start of world war 1?

The system of alliances contributed to WWI because Germany made a series of alliances to isolate France and avoid the country to take revenge from the Franco-Prussian war. Most of the Alliances were made in secret this made suspicions raise between pairs. This series of alliances divided Europe into two main groups - The Triple Alliance (Germany, Austria-Hungary, and Italy) and the Triple Entente (Russian Empire, France and UK)

might be agreeing to participate ethically with other european nations in the slave trade

or notifying other european nations when taking possession   of african territories