Alpha Omega
History and Geography—Two World Wars
Here's a combination textbook/workbook that examines the conditions in Europe that lead to World War I. Ideal for homeschoolers as well as classroom use.
Khan Academy
Khan Academy: Ap Us History: 1890 1945: The Us in Wwi: The Treaty of Versailles
Looks at the Treaty of Versailles, what its terms were, why America never ratified it or joined the League of Nations, why Germans grew to resent the burdens it imposed on them, and how this contributed to Hitler's rise to power....