How did World War I undermine the myth of European invincibility?
World War I did not directly call into question the myth of European invincibility in the same way that the Russo-Japanese War did. In the Russo-Japanese War, an Asian country defeated a European country in direct combat. This sort of thing did not happen in World War I. All of the major combatants on the winning side were European (or of European descent like the US). Japan did enter the war, but played only a very minor role. Therefore, the war did not prove that Europeans could be defeated by non-Europeans.
However, it did undermine the myth of European invincibility indirectly. It did this by undermining the idea that Europeans were intellectually and morally superior to other people. If the Europeans had been so superior, they would not have gotten themselves involved in such a bloody and pointless war. If they had been so superior, they would have managed to find a way to avoid all the terrible killing that happened in this war. Because the Europeans could not avoid this disaster, it became clear that their civilization was not necessarily superior to that of other areas of the world. It therefore followed that they might not be invincible.