In order to understand the principal interest of European princes, the negotiator must add to the knowledge which we have just been describing that of dynastic genealogies, so that he may know all the connections and alliances, by marriage and otherwise, between different princes, for these liaisons are often found to be the principal causes of conflict and even of war. He must also know the laws and established customs of the different countries, especially in all matters relating to the succession to the throne and the prevailing habits of the court. The study of the form of government existing in each country is very necessary to the diplomatist, and he should not wait until his arrival in a foreign country to study these questions; he should prepare himself beforehand, for, unless he is equipped with a certain measure of this knowledge, he will be like a man at sea without a compass. Our own negotiators, who have never travelled before taking up some foreign post and who therefore know nothing of these questions, are usually so saturated in our own national customs and habits as to think that those of all other nations must resemble them; the truth being that the authority which one king has within his kingdom in no way resembles that of the neighbouring monarch, although the superficial likeness between royalty in every country is obvious to every eye. [Sidenote: _England and Poland._] There are, for instance, countries where it is not enough to be in agreement with the prince and his ministers, because there are other parties who share the national sovereignty with him and who have the power to resist his decisions or to make him change them. Of this state of affairs we have an excellent example in England, where the authority of Parliament frequently obliges the King to make peace or war against his own wish; or again in Poland, where the general Diets have an even more extended power, in which one single vote in the Diet may bring to nought the all but unanimous resolution of the assembly itself, and thus not only defeat the deliberations of that assembly but bring to nought the policy of the King and of the Senate. Therefore the good negotiator in such a country will know where to find the balance of domestic power in order to profit by it when occasion offers.