Let's forget about the foreign enemy at first because under current circumstances that seems an unrealistic scenario for the next 75 to 100 years at least.
If we imagine a situation in which the US government suspended constitutional rights and declared martial law - for whatever reason - and the crisis that sparked this action ended (or was simply fabricated inthe first place), but the people in power were unwilling to return to the rule of law, this is what the 2nd Amendment is for.
Now the US government would still have access to the full arsenal of the US military. It is a force that no other military can currently match by a long shot. Therefore it can not be expected that the American people maintain an equally robust military capable of defeating our own military force.
One would hope that soldier themselves would be unwilling to kill American civilians, but you know what Abe Lincoln said, "you can fool some of the people all of the time." However, as we are seeing in Iraq, it does not take a modern, high-tech military to make it impossible to control even a relatively small country if the citizens are 1. unwilling to comply and 2. adequately armed.
IEDs and basic assault rifles are all that is necessary for the chaos in Iraq.
Now if we transpose a similar "insurgency" vis a vis Red Dawn in the US with citizen pitted against a rogue government, it would not take a force capable of actually handing a conventional military defeat on Us forces, but rather an insurgency that could not itself be defeated by those forces.
Thus there is no requirement for an offensive capacity, but rather only defensive. American citizens do not have to "take" Washington, but simply prevent Washington from "taking" them.
Come to think of it, this is true even under current peaceful circumstances.
I would very strongly suggest that you do not completely disregard the possibility. All it would take would be for a Quisling government- and we have lately seen recently that there is no shortage of globalist renegades in either major party- to cede several southwestern states *back* to Mexico, and attempt to enforce the reconquista with U.N. *peacekeepers.*
In such an event, you can also depend on a very high liklihood of US troops under UN command being involved as well.
And three, entirely ruthless and resolute in their desire to prevail.
In the example of the 4-month Finnish Civil War of 1918, about 40,000, near 1 percent of the population of Finland, were killed, but less than a quarter of them were on the battlefield, most during postwar retaliation and executions of those found to have been treasonous. The US managed about a 2% fatality rate during the US *Civil War* of 1861-'65, but that was spread across an overall period of several years.
The current US population is right around 300 million. You can figure for yourself what a 1% fatality rate would work out to. Or worse.