Some have questioned whether the Second Amendment is still useful as a measure against tyranny. Of what use are rifles and pistols against tanks, jet planes, artillery and the rest of the might of a modern army? It is a sensible question. It is sensible, at least, if it is honestly asked, without a snarky presumption that the conclusion is foregone.
Hard, grim and costly such a conflict surely would be. But hopeless? The first indication that it is not hopeless is every tyrant's insistence on disarming the people he means to oppress. It is a consistent habit shown by oppressors throughout history, including recent history. If the householder's rifle is no great threat, why take it away from him? For his own good? Hah.
Enough willing people with small arms can soon make a country ungovernable. That is a good start on removing an oppressor. It is not certain that the resistance would permanently be limited to small arms. In the Chinese Revolution, the Communists got much of their heavy weaponry through capture and from defectors from the Nationalist army. In this country a resistance movement would soon make use of any military equipment that became 'liberated.' We are as a nation pretty savvy about machines and technology. If we didn't understand how some weapon worked we would soon figure it out.
The idea would not be, and should not be, to take up small arms and go head to head against tanks and so forth. Any weapon is only reasonably used on targets it can effectively engage. The armed citizens' role would take advantage of their omnipresence within the culture and their knowledge of its day to day workings. The tyranny's petty administrators and their thugs and collaborators would be logical targets. So would enemy supplies and information channels. Direct confrontations with enemy forces would need to be carefully planned and of limited scope and duration, and target their infantry.
The idea of the Second Amendment is to make the right to gun ownership very nearly universal, with only those exceptions clearly needed, such as those for criminals and the mentally disturbed. This creates a situation where any oppressors, or potential oppressors, are surrounded by armed citizens wherever they go.
It is the underlying assumption of democracy that the good people outnumber the bad. The hope is that the assumption would hold true in a national emergency involving guns and tyrants. The Second Amendment, in other words, is democracy as it applies to the use of guns. It democratizes the use of force rather than leaving that the prerogative of the regime.
As per Patrick Henry, “The great object is that every man be armed…. Everyone who is able may have a gun.” Samuel Adams said “Constitution shall never be construed to authorize Congress to infringe the just liberty of the press or the rights of conscience; or to prevent the people of the United States who are peaceable citizens from keeping their own arms.” I would not go so far as to say "never." There have been some infringements in at least two of those areas. But the ideal is plain and so is its purpose: If the worst happens, we the armed citizens outnumber them, the oppressors.
At some point we could at least hope that there would be organizing efforts, proceeding from the state governments, to raise militias to reinforce the efforts of resistance cells. More is possible with in-depth organization and training than without it, thus the constitutional language that guarantees the states the right to raise "well-regulated" militias for their security. I am not sure what a modern state militia operating in resistance warfare would look like. I would like to think it would use all the modern war equipment it could make, find, scrounge, salvage, improvise or liberate. Beyond that stage we might conceivably hope to assemble a new Continental Army.
A point to ponder in thinking about all this is that it is easier to destroy than to build. That works in the favor of a resistance movement. A few rifle bullets, costing little, can disable a sophisticated plane worth millions. The trick is to shoot the plane while it is stationary on the ground. Tanks need fuel to go anywhere. Fuel can be made to burn in its depots. A great deal of damage can be done by asymmetric tactics such as these. The main limitation on them is running out of manpower to carry them out. If we are all dealt into the game, and the Second Amendment intends just that, by giving every one of us the right of resistance, we have a lot of manpower. Enough? Let us hope we don't need to find out.
It may be--and this is my hope--that an armed citizenry is simply too much trouble to be worth oppressing. Those with tyrannical ambitions will shelve them so long as the right to keep and bear arms is in place. Thus the Second Amendment serves its purpose even if we never need to put it to the test. It stands as a threat to any tyrant that we are, if sufficiently provoked, able to turn the country upside down and be ungovernable. It gives real weight, as nothing else can, to the phrase "consent of the governed."