Resources for Causes:
WikiBooks - Covers long- and short-term causes, the initial parts of the war, and the United States entry into the war.
BBC: The Origins of World War One - A short essay looking at the factors contributing to war.
First World War - An essay on the start of the war.
General Information about the War
BBC Archive with all their information on World War I
World War I in 6 Minutes (See below) Link: https://youtu.be/-3UjJ5kxiLI
Causes
General
http://www.hyperhistory.com/online_n2/History_n2/a.html - select events from the right and then find WWII in the list on the left
https://en.wikibooks.org/wiki/World_History/Causes_and_course_of_the_Second_World_War
10-minute History: Russian Revolution - Be sure to read the slides as you listen and watch. They contain additional information.
Epic History: The Russian Revolution - The timeline at the top is a nice feature that helps track the developments.
Russian Revolution - This PDF is of the PPT that had the rock-paper-scissors description in it. The PDF has some issues with items covering up earlier images. The Slides version still needs to be modified to look right.