Historians still quibble about what event marked the end of the Roman Empire. Some future historians might choose Jan. 16, 2021 as the fatal moment for the American Empire. That’s when the People’s Bank of China and SWIFT, the global system for international monetary transfers, set up a joint venture to promote the use of China’s digital currency in cross-border payments. It’s early days, but the joint venture could denote the end of the dollar’s role as the dominant medium of international exchange, and, more importantly, the end of more than $20 trillion of cheap loans to the United States...