2. Differences: The first cooperation was intra-party cooperation, which basically overthrew the rule of Beiyang warlords, but it broke down halfway. The second cooperation was non-party cooperation, which defeated Japanese imperialism and persisted until victory.

3. Reason: The Japanese imperialist war of aggression against China has gradually expanded, and the ethnic contradiction between China and Japan has become the most important contradiction in Japanese society, and the Chinese nation is in serious danger of national subjugation.