Why was the Treaty of Versailles effective?
The Treaty of Versailles was not effective. It completely failed to do what it was supposed to do.
The major goal of the treaty was to keep Germany from threatening the peace of Europe again. It was supposed to do this by weakening Germany in a variety of economic and military ways. However, the treaty did not succeed in keeping Germany down. It was not effective because England and France were not willing to enforce the terms of the treaty. When Germany started, under Hitler, to violate the treaty, the French and English felt unable to do anything about it. They gave in to Germany over and over again, helping lead to WWII.
Therefore, we cannot really say that the Treaty of Versailles was effective.