What part did the alliance system play in the events that led to World War I?
a. Countries that were in alliances with other countries were obligated to support their allies when war was declared.
b. Alliances enabled countries to cooperate in establishing large colonial empires.
c. Countries that were in alliances with other countries were guaranteed that they would not have to go to war.
d. The countries in one alliance system were not allowed to trade with countries in competing alliances, which further divided Europe.