In his short story "The Last Article", Harry Turtledove posits a very different outcome resulting from mass defiance.
He examines a fictional period in which the German Army of WW2 has invaded India. The people of India respond by adopting a policy of mass non-compliance. A little bit of blood and iron quickly whips the Indians back into obedience.
I would think that Turtledove's outcome is perhaps more likely than yours ... that the IRS would just "pack up and go home".
The cultures of 1940's India and the U.S. today are drastically different. A foreign invasion would immediate be met by widespread, violent resistance, not "peaceful protests."