Every era has a hegemon, and in our time it is the US.Every hegemon throughout history has maintained that position by forcing other nations to use its currency for trade. (as well as helping themselves to those nations' resources.)To enforce this, it must maintain the largest military.
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The USA doesn't force anyone to use its currency. They use it because it's the reserve currency and recognised everywhere.
You really think I'm going to head off for a sojourn overseas with a wallet full of worthless yuan or even worse...roubles?
I go into a restaurant in Italy and break out the roubles to pay and I'll get thoroughly rubbished.