The last chart on this page shows how much of US Treasury Debt is bought by foreign entities, the Fed and us little guys (through whatever retirement accounts we have). When Americans are tapped out - or stop having faith in the gov - a default is imminent unless the foreign purchases start going back up.
https://www.zerohedge.com/news/2019-06-12/who-really-funding-uncle-sam
https://www.zerohedge.com/news/2019-06-12/who-really-funding-uncle-sam