One Perspective On Government
Some libertarians oppose governments on the principle that they are organised gangs of thugs. They consider the defining characteristic of governments to be that governments claim the right to initiate force ... and people listen (whereas most thieves don't pretend to be legitimate and aren't considered as such). They point out that they never agreed to pay taxes, and don't want to, and don't like most of the stuff that taxes pay for, and consider that conclusive.
Some of these libertarians support the war on terrorism. They realise that terrorism is a great threat, and to wish see it fought against. Terrorism is so bad that anyone at all fighting it is good. I suppose they must see the matter as a powerful pickpocket guild beating up a renegade gang of murderers. A "lesser of two evils" situation.
(Some libertarians would oppose the war on terror, either because they figure "If we leave them alone, they'll leave us alone, and nevermind Israel," or "No collateral damage is ever acceptable, under any circumstances, for any purpose, even if it is only caused because the enemy is using human shields." But I won't go into how silly I think those approaches are right now.)
Some of these libertarians, if given the option, would be happy to see the US government disappear tomorrow. The institution, the knowledge of how to run it, the taxes, the laws, etc... This is absurd, notably, even within the pickpocket metaphor, as it means foregoing protection.
But there's more than that; there are good reasons to like our government and support it besides self-defense. Our government does various things, some important. Now, the libertarians will insist that all these functions could, in theory, be done by private companies. Well, yes, I agree. But so what? I don't see these companies. They don't exist (yet).
It's not as if an anarcho-capitalist society (in short: free market capitalism with all government functions replaced by private companies and taxes replaced by user fees for people who want the services) would simply come into being without our government. Anarcho-capitalism is not the natural state of affairs that once existed until it was destroyed when a group of evil thugs invented government and took over. It is, rather, a very advanced notion that requires lots of knowledge to implement. This knowledge must be created gradually, through the improvement of existing institutions. Government functions must not disappear overnight, but instead slowly be replaced by private institutions that function better. We need good traditions, not a revolution.
Why Government Is Good
Governments create consent. That's the reason in a nutshell, but of course it needs an explanation.
Let's imagine a group of people living somewhere with no government, and little knowledge. Some will be bad, and will want to dominate over the others. So most people will form mutual defense pacts. And somewhere not too far off, some bad person will have conquered an empire, and formed an army, and thus our people will want to form one big defensive pact, instead of lots of scattered ones, so that they can fend off the entire army if need be. So they will form institutions to cooperate in regional defense. When an invasion looms, there may be disagreements about how many soldiers are needed to fight it off, and who must become a soldier, and where their equipment will come from. Thus, a system to resolve these issues is needed.
And these people will also set up institutions for small-scale defense against criminals. And they will need some system of deciding who is and is not a criminal. The answer to this is not self-evident despite what some libertarians seem to think. There will be disagreements, and thus some way to resolve them will be needed.
One day, Joe's crop goes bad. He asks others for help. They form some food-sharing institutions. They create rules to govern these. The people all value security, and thus put in provisions to help anyone who does not have enough.
One day they invent medicine. They realise that if they only pay the doctor when they are sick, he will starve in the mean time. And also that he will have no motivation to help prevent people from becoming sick. So everyone pays a low price all the time, and the doctor helps whoever needs help at recovery and prevention both. Some people disagree about who the doctor should be helping, saying he favours his friends, and they create institutions to resolve disputes of that nature.
What will all these institutions look like? Well, at first they will be very crude. The defensive agreement might simply state that all able-bodied men must fight when there is a war, or be put to death. The food agreement might allow anyone who is starving to take food from his neighbor, "as long as he made a genuine effort to create his own food." And the system of resolving disputes might be to ask the town elder.
And, over time, people will come up with better ideas. And after a while, and a lot of progress, something like our current government and courts might form.
If this society (that we've imagined) progresses to use a completely voluntary army, that will be an amazing advance. And if it has elected leaders who consent to voluntarily step down when their term ends, that will be an amazing advance. And if criminals are presumed innocent until evidence is presented against them, that will be an amazing advance. And if there are property rights defended by law, and a system of consensual trade, that will be an amazing advance.
When we know how to do better than using government for these things, we will. But we do not. The path to a better society is not to rail against our government, but rather to acknowledge it for what it is: an imperfect, evolving tradition and a great force for good.