The Triffin DilemmaFrom Wikipedia: "The Triffin dilemma or Triffin paradox is the conflict of economic interests that arises between short-term domestic and long-term international objectives for countries whose currencies serve as global reserve currencies. This dilemma was identified in the 1960s by Belgian-American economist Robert Triffin, who pointed out that the country whose currency, being the global reserve currency, foreign nations wish to hold, must be willing to supply the world with an extra supply of its currency to fulfil world demand for these foreign exchange reserves, leading to a trade deficit."
So a country whose currency serves as the global reserve currency has to simultaneously import more than it exports in order to keep the world liquid with its currency while at the same time export more than it imports to keep the currency strong. Obviously it can't do both at the same time. Thus the dilemma.
The US has been resolving the dilemma by running trade deficits to keep the world liquid in dollars and acting as the world's policeman to keep its currency strong. The US military plays a huge role in backing the US dollar. If they ever failed to keep the peace, then the dollar would weaken. People would lose faith in the US.
So Russia had better think twice about invading the Ukraine because Uncle Sam will never accept challenges to US dollar supremecy. Also, the US is "too big to fail" in that if the dollar ever got decimated, then the whole planet's financial system would come down with it. So the US would find plenty of allies to help contain Putin's ambitions, given that something like 80% of global central banks reserves are US dollar denominated which is why the US dollar is called the reserve currency. If it goes down, then so do most other currencies.