See, this is the sort of question that can lead to undesirable answers.... some of which are clearly demonstrated in these comments. If your answer is premised on defending "Buddhist countries" from Christian or Islamic influence, that is a textbook example of begging the question. Do Buddhist countries exist? Should they?
Are you willing to be consistent with your answer? If you are willing to equivocate about extremism in Burma, would you also countenance Americans sponsoring revolutionary violence to "liberate" Tibet from China?
Are Buddhist countries even necessary? Is it possible that Buddhist monks would be just as protected, better protected, by multicultural parliamentary democracies?
Is a wheel-turning monarch necessarily even a monarch? Is it possible that the wheel turning monarch is merely an aspiration, a metaphor for rule of law, social progress, human rights?
Isn't it obvious that all past and present claimants of a wheel turning monarch have fallen short of the ideal? Can we agree that the texts pertaining to the wheel-turning monarch are still useful, and authentic?
The Catholics have the idea of a just war, and one of the requirements is that it must be likely to succeed. But Buddhism teaches impermanence and suffering; a defensive war or war of liberation is guaranteed not to succeed, at least not permanently. In the Four Summaries of the dharma, Rathaputta questions the king and the king admits that, yes, he will lead his nation into an aggressive war of conquest in all four directions should the opportunity arise. Anybody who is strong today will be weak some day. Nobody gets to keep their money or land forever.
In Buddhism, God is not watching; there is no assurance of justice in this life, or justice for any specific individual or group of individuals. It's all about long term trends, eons, the law of averages.
Therevada Buddhism teaches against violence, wielding power, owning land or possessions. Nonviolence is one of the five precepts. In Mahayana Buddhism, rules are made to be broken. The Pure Fame Sutra says that the Bhoddisatva can violate the five precepts; when be goes to hell, his mind is unmoved by agony or distress.
Deitrich Bonhoeffer was a perfect example; he, as a Christian, believed in authority and nonviolence, but he attempted to kill Hitler regardless. He was willing to go to hell if it would end the war and save Jewish people.
The object of Buddhism is not to decide if killing is permissible in certain situations, and then strategize accordingly. It's about, how will you maintain compassion and tranquility even when there is no possibility of justice or recompense? How will you set an honorable example, for friend and foe alike, not for your benefit but for theirs? If you miscalculate, and you will, can you face the consequences with courage?