Weapons can be an endless pit of development and the US is not a conquering nation. Let's say we could sustain an economy and develop the first nuclear weapons while everyone else is using spears; what was the opportunity cost? What's the value if it's only used for defense?

Outside of defense I know a large military and weapons help in having influence, but so does having a stable and large economy--the later moreso.

Nuclear weapons were developed out of a competition for survival. The people involved knew even in the hands of the Allied forces it was extremely dangerous. You're arguing we should invest in a Space Force because someone else might do it in the future.