Answer: Germany
The Treaty of Versailles was the main treaty that ended WWI. It was signed on June 28, 1919, exactly five years after the assassination of Archduke Franz Ferdinand.
The treaty was extremely harsh to Germany. It required it to accept the responsibility for all the loss and damage during the war, disarm, make many territorial concessions and pay extremely high reparations. Germany was weakened, but not pacified or conciliated, and some scholars argue that the harshness of the Treaty of Versailles was one of the catalysts for WWII.