How were the alliance system, nationalism, and Kaiser Wilhelm the most important causes of WWI?
These three factors are among the most important causes of the war. Let us examine each of them.
Nationalism led to the war largely because it helped to cause strife within Austria-Hungary. It also connected that strife to other countries. Austria-Hungary was an empire ruled by Germans, but which had many Slavs among its subjects. Slavic nationalism led those Slavs to want independence. Nationalism got Russia (a Slavic country) involved in supporting their struggle. It got Germany involved in supporting its fellow Germans.
Alliances led to the war because they made everyone feel unsafe. (They also helped spread the war once it started.) For example, when France, England, and Russia formed an alliance, Germany felt that it was surrounded by enemies. This made its leaders more convinced that war would be necessary.
Kaiser Wilhelm contributed to the war because he was the driving force behind Germany’s aggressive actions. When he came to the throne, Germany was not a first-rate power in the world. He wanted Germany to have its “place in the sun.” Therefore, he started doing things, like increasing Germany’s military power, that frightened the other countries in Europe, leading them to form alliances against Germany.
In these ways, the three things you mention helped to bring about WWI.