The United States government is guaranteeing mortgage backed bonds purchased by China. Those are worthless bonds unless their value is assured by the US government. When did the United States government get involved in Wall Street’s casino? That most likely happened when the feds decided to rescue rich investors who’d made mistakes in purchasing the bonds.
When the housing market tanked, the bazillions of dollars of mortgage backed bonds held by the affluent plummeted in value to right around zero. The Federal Reserve purchased over a trillion dollars of these valueless bonds at face value, dollar for dollar. They saved foolish, hapless, stupid, but rich investors.
Apparently, the federal government now guarantees those bonds like they do student loans.