The Treaty of Versailles was a peace treaty that ended World War I. It was signed on June 28, 1919 and officially went into effect on January 10, 1920.
The Treaty of Versailles placed the blame for the war squarely on Germany and forced it to pay reparations to the victorious allies. It also formally ended the war between Germany and Russia, Austria-Hungary and Turkey (which had already been defeated by then).
The main reason why this treaty was unfair is because it did not take into account all of the factors that led to World War One. For example, France and Britain declared war on Germany after Germany invaded Belgium and Luxembourg in 1914. However, France and Britain had both been actively arming themselves against Germany for years before 1914. In addition, Russia had been building up its army since 1912 and had just signed a secret treaty with France two weeks before war was declared on Germany.
In addition to being unfair because it did not take into account all of the factors that led to WW1, this treaty was also unfair because it punished Germany unfairly by placing all responsibility for WW1 squarely on their shoulders. This led to resentment among Germans who felt as though they were being blamed for something that was not their fault.
The treaty was extremely harsh towards Germany, which led to its government becoming increasingly unstable and eventually overthrown by Adolf Hitler in 1933. This led to World War II.