Just because the dollar is the worlds biggest currency doesn't mean that it doesn't need a strong military. Countries that are anti-America, or just anti-dollar, will oppose that for example Iran or Russia has a big military yet its currency is nowhere as big as the Us's. However, China on the other hand has a large military and its currency may not be the worlds, but its economy is rapidly growing and i believe that this question should focus on the growth/decline of countries' economy's
The reason the United States is a reserve currency is because it has a strong military and won out in every recent geopolitical conflict.
We do not need to have a strong military if we do not want to be the world's reserve currency, and there is no inherent need to be the world's reserve currency. It's certainly nice, but we don't 'need' it.