Answer:
they could not defend themselves against stronger groups.
Russia's victories in World War I led to Czar Nicholas's abdication and retirement. War on the western front devastated Russia's army, causing Czar Nicholas to abdicate. ... Czar Nicholas abdicated in favor of constitutional governments due to Russia's alliance with Britain and France.
Rebelling ethnic groups is 100% correct
The United States of America joined World War II when Pearl Harbor was bombed...if you're referring to the 2nd World War.