The number we really need to look at is deficit, because such large endeavors pretty much always tend to involve some form of debt (paying investors, securities, benefits, etc.)
When they got into trouble is when they started to run a deficit - debt exceeds revenue and creates more debt.
We could conceivably run a debt since securities are paid out on a schedule, and still be in the black. This means there's still an obligation to pay holders of securities, but still gaining money rather than losing it. That isn't necessarily a problem.
When we consistently run a deficit though, the problem is two fold - those who hold securities get nervous that the government might print money to pay them off, which lowers the value of the money. We definitely want to try and avoid doing that because if people start selling off their securities, the value of our currency falls.
The second problem would be dealing with the deficit in ways that can otherwise hurt the economy itself, such as the austerity we keep hearing about. That has to be avoided too or otherwise the value of money falls for a different reason (e.g. unemployment and lack of circulation)