It is a highly speculative situation, bofin. In principly you might be right that A could invade B and still be a polyarchy, but it would require that the inhabitants of A respected the civil rights that all people of A share, but were willing to ignore these rights for the people of B.

I only think that this would be psychologically possible if the people of B were not considered human. We see that democracies are perfectly able to slaughter large populations of cattle to protect from mad cow disease. However, the premise of polyarchy that alternative sources of information must be available will make it very unlikely that a democracy would actually be able to commit genocide on another country.

qanatoli