Some historians write that World War I (the Great War) was avoidable. What institutions and ideas developed after 1919 to prevent a recurrence? Why did they fail by 1939? How did World War I change the face of modern warfare? Did the warring parties violate or ignore understandings about tactics and weapons? Were any international agreements violated or ignored?

International agreements have a mixed record of success in terms of limiting or preventing dangerous behavior by individual nations. How can this be improved or changed?