Many libertarians, after Rothbard, start from the (correct) assumption that one's government is one's first and most direct enemy, to the conclusion that one should always side with the enemies of one's current oppressor. Rothbardians have thus prolificly denounced the US and supported its enemies in its hot and cold wars with National Socialist Germany, International Socialist Russia, Communist China, North Korea, North Vietnam, National Islamist Iran or Iraq, etc.
Of course, applying the same "logic", the respective citizens of those countries whose government are in conflict with USG should in turn support the US government in its fight against their own — if only their own government wouldn't murder them immediately at the mere utterance of such a support. And to take this line of reasoning to its conclusion, a Pole in 1939 should have supported Hitler and Stalin as opponents to his current oppressive government.
A "logic" that reaches different conclusions for different people is actually not a logic. It's polylogism, a fallacy of double standards, a rhetorical device to back whichever absurdity one fancies. Moreover, underlying this fallacy, we see another typical case where people who should know better fall into an accounting fallacy: just because a current oppressor is identified (current account negative) current non-oppressors (current account zero) are considered a better alternative as part of an unrelated future choice between oppressors.
"There are two kinds of pacifists: those who try to disarm the aggressors, and those who try to disarm the victim." At the margin, you may only have the choice between two oppressors. Making this economic (moral) choice about the future based on a historical accounting of one's past personal relationship with them is completely stupid and baseless. The enemy of your current oppressor may oppress you far more than said current oppressor if he wins, not to speak of his current victims, as Hitler and Stalin may have amply demonstrated to the hypothetical Pole who would have believed the rothbardian argument.
Some oppressors are objectively better than others. And one should support the better oppressors against the worse oppressors, whichever of them is currently oppressing one. For all its warts, the corrupt US-backed South Vietnamese regime was better than the wanton mass-murdering Russia-backed North Vietnam, and should have been supported in its war of defense against the communist aggressor. The wanton mass-murdering Russia-backed North Vietnam itself was better than the outright genocidal China-backed khmer rouge, and should have been supported in its war of aggression against that regime. The latter regimes were no less imperialist than the previous for being backed by Russia or China rather than the US or France; but they were much less libertarian.
Similarly, for all the crimes committed by their men, the British and French Colonial Powers should have been supported in their conquests of barbarian and totalitarian powers that previously existed in Africa, India, Vietnam, etc. In all those countries, colonial oppression may have been a bad thing, pre-colonial oppression was worse, and so was post-colonial oppression. The regimes that were toppled by colonial imperialists were never peaceful to their own people, and had no "right" to remain in place. The offenses they committed against their later victors may have been large (as in the case of systematic muslim raping enslavers around the mediterranean sea) or comparatively trifling (as in the case of various disputes with european merchants or missionaries); they were often reason enough to wage war, and even when not, the new conqueror had no less right to rule than the previous one; only more so for his more liberal laws.
But apparently, to the racists who call themselves anti-colonialists, the color of the skin of the ruler matters more than whether he's an honest, competent administrator or a corrupt wanton mass-murderer. And so the racists and leftists from the Western Establishment, after conquering the University, the main-stream media and the public administrations, have managed to push for local brown-faced mass-murderers to replace rather honest white administrators everywhere that a colonial power once existed, tamed after its initial bloody installation. Worse, they have successfully disarmed and embargoed those who were resisting communism in Russia, China, Vietnam and paralyzed the attempts of western reactionaries to help the victims.
Let it not be said that those of us who call for the use of force against barbarians are pro-war. The war was started long ago, and the enemies of life, property and liberty have never granted any truce even less peace, to those they seek to oppress. Those who deny property rights and claim totalitarian power over our lives are at war with us, whether their agenda is open or remains concealed behind pleas for greater "social justice" and claims of "saving the planet". They have successfully subverted many weak societies and their entryism in all venues of power is now bearing fruit even in the formerly strongest societies. Expect no mercy from these enemies. And grant them none.
The essential proficiency in political science is to distinguish friends from foes. You may be a fool and not know the difference; the enemies of liberty are quintessential political beasts and sense this difference very well. You're a fool if you believe yourself at peace with socialists, islamists and other totalitarians. You're a fool no less if you realize there is conflict but think you can bargain, negotiate or compromise your way into peace with them. Any appeasement talk with totalitarians can fool but yourself and your allies, never the enemy. Their very nature, the way and the reason they live, their deepest aspiration, their ultimate value, is to prey on producers; there is no more peace to be had with them than with the plague. If you're waiting for men in black to knock at your door before you realize these men are at war with you, you're a pretty useless ally in this war; you're just another of these "useful idiots" as Lenin called them.
Free Market defense agencies would certainly have to fight attempts by the totalitarians to seize power; meanwhile totalitarians would certainly do systematic entryism in whichever defense agencies they can to turn them into their tool. And so this eternal war will certainly continue in the Anarchy dreamed by anarcho-capitalists just as it exists now, though freedom to choose one's defender may make it easier to win that war over and over again. In any case, fighting back at the totalitarians will happen in the Free Societies of Tomorrow, and is thus certainly no crime. It thus isn't a crime now even in the Semi-Free Societies of Today, and wasn't a crime in the Semi-Free Societies of Yesterday, though those who may have waged this war may themselves not have been innocent of other lesser crimes.
However, inasmuch as there are many tyrants to be kicked out in the world today there is no more an Imperial force to kick them out and replace them with something better. Inasmuch as there are a whole lot of totalitarian activists to be crushed, there is no Imperial force to crush them and spread civilization in place of the poisons these activists spread in people's minds. "Conservatives" and "Neo-conservatives" have bitten bait, hook and line into the "progressive" ideology of Democracy, and will sacrifice all liberties on the altar of this God That Failed. They cannot replace tyranny with anything but another tyranny in foreign countries, and even at home they are part of the problem rather than the solution. Meanwhile, "progressives" and outright "socialists" continue with little impediment on their war on liberty, and there is no one to stop them, only minor hurdles that slow them down. National armies and police forces, having monopolized the defense of property rights in the last semi-free countries able to fight, are now securily controlled by enemies of liberty. The war has been waged, and it has been won by the Enemy.
But this war was waged first with ideas. The reconquest will also happen first with ideas. Before guns may speak on our side again and at long last give the totalitarians a taste of their own medecine, we have a long way to go. Happily, modern technology makes it easier than ever for ideas to spread; and so, we must wage the war with ideas, with faith that it is precisely what makes our ideas good that will make them prevail in the end.