The IMF lists a 125% debt-to-GDP ratio as "unsustainable". The US just passed that mark early this year. We were mostly at about 40-60% for the past century or so (WWII being a huge and understandable exception). When Obama took office, it was 64%, which was about average for the 220+ nations on the globe. We are now #11 in the world, at 125%. Those above us include (no surprises here) Ireland, Greece, Spain, Italy, Portugal, Sudan, Zimbabwe, and a few Caribbean Islands... but #1 by a long shot is Japan, at over 210%. When interest rates rise by just a few percentage points (and someday they will), Japan will be spending a HUGE portion of their tax revenues just to service the debt. That's when things start to spiral... and that is why interest rates have not been allowed to go up for several years. Most of the largest economies have been throwing Trillions of dollars away all to keep those rates down. It won't last forever... and nobody, not eve a healthy and robust US economy, is big enough to bail out Japan.
(Greece has a GDP of a quarter Trillion. Germany is at 3.5 Trillion. Germany has kept Greece afloat, because they can. They are 14 times bigger. Japan is at 6 Trillion. The US and China combined are only 24 Trillion, only 4 times larger... and China's willingness to help Japan surely must be questionable.)
Be prepared, folks