The Federal government’s weakest link is the IRS. Any disruption of revenue will destabilize funding programs and the currency. The Treasury’s ability to borrow and print is predicated on a certain base of revenue. If the ratio of borrowing + monetary expansion to tax revenue becomes too great, credit ratings will plumment, and creditors will demand higher interest or simply pull out of the US soverign debt market. More money printing will only add fuel to the fire.
This could happen if the economy stalls and goes into a severe recession, or there is some sort of civil disobedience/strife, black markets/evasion etc.
Once civil strife hits, the disobedience will likely increase, but IRS enforcement will intensify. States could pass laws to protect citizens from SWAT raids by the IRS, but I doubt it. The situation may deteriorate to the point of martial law (especially in blue states and cities). Then, at that time, we may see targeting of IRS agents, auditors, their families etc. Capturing a high ranking IRS IT guy could lead to hacking of IRS computers and widespread malfunctioning of the collection system.