Why does the USA need a defense budget so large?

I’ve seen many graphs and much information regarding defense spending by the USA. Here’s one place for information. Spending over $700 billion annually, it states that the USA accounts for 41% of defense spending for the planet. In second place, China accounts for a very distant 8%, spending only just over $140 billion.

Do Americans really need to spend so much? Really?

Please note, this is a General Section question. Answers and debate should relate to the question and the details.