This, and the premise in the OP, is demonstrably, historically false.
We have at least one *very* good example, very recently, showing how decentralized nations are *extraordinarily* difficult to 'conquer'. Let me describe it. First off, the 'country' is certainly not libertarian - but that's not the point here. It's a centralized and focused military power against a decentralized society.
The invading nation has *SIGNIFICANTLY* greater technology, tactics, and experience in organized warfare. It is militarily centralized, and focused. It's backed and funded by an economic superpower with seemingly limitless resources compared to the nation being invaded. The invaded nation does not have particularly enforced borders (is basically open borders), and is a third-world nation under an agrarian society, and still mostly using technology from the middle ages (some are able to use crude but still inferior modern weapons and improvised explosives for guerilla warfare, however).
But for some reason, the vastly technologically, militarily, and economically superior invading nation is not able to effectively conquer the other nation. As a matter of fact, the war has become increasingly expensive (financially and politically), contributing to bankrupting the invading nation, demoralizing the troops and the people in supporting the war, and attacks against the invader have become increasingly common and more effective as time has gone on.
Anyone know the example I'm giving? Give up?
It's the USA vs fucking Afghanistan.
And this isn't even an economically and technologically advanced libertarian 'nation'.
/thread