Answer:
Competition to colonize smaller countries rose.
Countries were engaging in militarism.
Countries were splitting into alliances.
Smaller countries rose up to fight imperialism.
Explanation:
Competition to colonize smaller countries rose because some countries, like Germany and Italy believed that division of colonial power was unfair.
Countries were engaging in militarism, because they believe that a strong army makes a strong country.
Countries were splitting into alliances and the strongest one were Central Powers and Entente.
Smaller countries rose up to fight imperialism and that is why four greater empires disappeared after the war.
Poison gas was used during the war, but is not a cause of it.