Final answer:
The Japanese invasion of Manchuria in 1931 prompted the opposing parties in China to unite against a common enemy, leading to the Nationalists and Communists working together during the Second Sino-Japanese War.
Explanation:
The event that motivated the opposing parties of China to fight together was the Japanese invasion of Manchuria in 1931. This invasion prompted the Chinese Democratic Party, warlords, and the Kuomintang to unite against the common foreign enemy, Japan. Despite their internal conflicts, the Nationalists and Communists joined forces to resist the Japanese aggression during the Second Sino-Japanese War.
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