There were numerous treaties negotiated during and after World War I (1914–1918). Also, a mandate system was set up after the war for the administration of former German and Ottoman territories. National leaders' predilection for secret accords during the war contributed to the unprecedented disaster that was World War I; President Woodrow Wilson of the United States wanted to end secret treaties and included his wish in the Fourteen Points. WWI was fought between the Central Powers (Germany and Austria-Hungary) and the Allies France, Britain, and Russia).
As soon as the two warring sides realized that the war would not be a short one, they both recruited allies by offering incentives via secret treaties. Italy signed the secret Treaty of London (1915) to become a member of the Allies. In 1916, Romania signed a similar agreement with the Allies. Italy and Romania were both promised Austro-Hungarian territory in exchange for their participation. The was was a disaster for both countries: Romania was overrun and forced to surrender and Italy suffered a huge defeat at Caporetto before rallying late in the war.
There were other secret accords. The Constantinople Agreement (1915) was between France, Britain, and Russia and it concerned the division of the Ottoman Empire. In 1917, Germany sent the Zimmermann Telegram to Mexico: it proposed a secret alliance against the U.S.
The Versailles Treaty (1919) was the main agreement that ended WWI. Germany was forced to accept sole blame for the war. It had to cede a lot of territory and pay heavy reparations.