In this task, you're given a story (which contains five sentences only). Your task is to find all the characters which are available in the given story.

Let me give you an example: Sentence1: My dog was bored. Sentence2: I was working all day so I couldn't spend time with him. Sentence3: When I finished work I decided to play with my dog. Sentence4: He was so happy to finally have some fun! Sentence5: Alas, he fell sick and died.
The answer to this example can be: I (myself), Dog
Here is why: There is only two characters in given story, i.e., I (myself) and dog.

OK. solve this:
Sentence1: Japan was building an Empire in Asia. Sentence2: The United States didn't support this. Sentence3: Japan needed to get America out of the Pacific. Sentence4: Japan decided on a pre-emptive strike. Sentence5: On December 7th Japan launched a surprise attack.
Answer:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
United states, Japan, America, Launched, Strike