Japan were not so much unhappy with the outcome of the treaty of Versailles as with how the treaty negotiations took place.
They felt probably correctly that they had been sidelined by the four major powers. However it should be noted that Japan didn't have any role in the European theatre of the war and its contribution was largely confined to providing convoy escorts and attacking Imperial Germany's possessions in the pacific.
For minimal effort during the war Japan was given what had been Germanys pacific Islands together with former territories in China.
The last part was hugely controversial the Germans had seized Shandong from the Chinese and the allies rather than return it to China gave it to Japan.
The Americans as well as some other allies used the venue of Versailles to make criticisms of the rather brutal way that Japan chose to run its empire. At this stage Japan controlled Korea chunks of China as well as various other smaller territories.
The Japanese never attempted to disguise the fact that they viewed other asians as racially inferior. The racial equality motion was merely an attempt to draw equivalence between their empire and the British and French empires.
<span>So at the time the Japanese weren't particuarly displeased at the versailles treaty. In later years it would become tied with the hated Washington naval agreements signed in 1921 which would lead to massive unrest in the Japanese military.</span>
WWI was caused by many things, among them was the assassination of Franz Ferdinand leading to the Austria-Hungary empire declaring war on Serbia which dragged most of Europe into the war due to alliances. The U.S. entered the war due to Germanys war tactics and the Zimmermann note.
