Why did Japan and Italy oppose the Treaty of Versailles?
Britain, France, the US, and Italy were the main power brokers at Versailles. While Japan was a member of the Allies, it was invited to the treaty table because the European power brokers realized that Japan was becoming a key player in the Pacific region. The two Japanese representatives at the treaty wanted their holdings in the Pacific recognized, but Britain and France were mainly interested in either gaining Germany's former colonies for themselves or creating mandates that would be governed by the League of Nations.
Japan's main desire was to add a provision of racial equality to the Versailles Treaty. The representatives thought that this would be no problem, since Wilson provided for self-determination in order to draw up national boundaries. Britain and France still had immense overseas holdings--there was no way that they could acknowledge that people living in the developing world were the equals of those who lived in Britain. Also, the US's African Americans lived under the shadow of Jim Crow laws in 1919, and Wilson was not in favor of racial equality. The Western powers at the Versailles table dismissed Japan's wishes without even giving them attention. The Japanese representatives did not leave the treaty table, but they were insulted. This would lead to militarists taking over in Japan and Japanese imperialism within the next twenty years.
The Italians, who were initially part of the old Triple Alliance, had entered the war due to promises of territorial gains from Austria. These agreements, made in the secret Treaty of London in 1915, posed a major problem after the war's end. In short, Italian nationalists were angered by the refusal of the Big Three, especially Wilson and Clemenceau, to honor these claims. Technically, the Italians were not as affected by the Treaty of Versailles, which concluded a peace with Germany, as they were by the Treaty of St. Germain-en-Laye, which made peace between the Allies and Austria.
Japan, on the other hand, was unhappy with the refusal of the other powers to accept a provision that included a recognition of racial equality among all of the members of the League of Nations. Like Italy, they were also unsatisfied with their territorial gain as a result of the war. They acquired some of Germany's island colonies as well as the Shantung peninsula, but wanted more territory, particularly in China.