Answer: Japan's bombing of Pearl Harbor in Hawaii.
Explanation: The event that this poster wants Americans to avenge is Japan's bombing of Pearl Harbor in Hawaii. In this poster, we can see a stout man with his fist ready to fight. Then, we can see a date in red, December 7. This date refers to Japan's bombing of Pearl Harbor in 1941. We can also see a burning ship that is sinking. This event led the United States to get involved in World War II. This poster made Americans angry and eager to fight and avenge the bombing.