This was a golden era for diplomats and international lawyers. To the network of alliances that became established throughout Europe during the Renaissance, the Reformation added confessional pacts. Unfortunately, however, the two systems were not always compatible. The traditional amity between Castile and England, for example, was fatally undermined when the Tudor dynasty embraced Protestantism after 1532; and the “auld alliance” between Scotland and France was likewise wrecked by the progress of the Reformation in Scotland after 1560. Moreover, in many countries, the confessional divisions of Christendom after Luther created powerful religious minorities who were prepared to look abroad for guarantees of protection and solidarity: for example, the English Catholics to Spain and the French, German, and Dutch Calvinists to England.

These developments created a situation of chronic political instability. On the one hand, the leaders of countries which themselves avoided religious fragmentation (such as Spain) were often unsure whether to frame their foreign policy according to confessional or political advantage. On the other hand, the foreign policy of religiously divided states, such as France, England, and the Dutch Republic, oscillated often and markedly because there was no consensus among the political elite concerning the correct principles upon which foreign policy should be based.

The complexity of the diplomatic scene called for unusual skills among the rulers of post-Reformation Europe. Seldom has the importance of personality in shaping events been so great. The quixotic temperaments and mercurial designs of even minor potentates exerted a disproportionate influence on the course of events. Nevertheless, behind the complicated interplay of individuals and events, two constants may be detected. First, statesmen and churchmen alike consistently identified politics and religion as two sides of the same coin. Supporters of the Bohemian rebellion of 1618, for example, frequently stated that “religion and liberty stand or fall together”: that is, a failure to defend and maintain religious liberty would necessarily lead to the loss of political freedom. The position of Emperor Ferdinand II (1619–37) was exactly the same. “God’s blessing cannot be received,” he informed his subjects, “by a land in which prince and vassals do not both fervently uphold the one true Catholic faith.”

These two views, precisely because they were identical, were totally incompatible. That their inevitable collision should have so often produced prolonged wars, however, was due to the second “constant”: the desire of political leaders everywhere, even on the periphery of Europe, to secure a balance of power on the continent favourable to their interests. It is scarcely surprising that, when any struggle became deadlocked, the local rulers should look about for foreign support; it is more noteworthy that their neighbours were normally ready and eager to provide it. Queen Elizabeth I of England (1558–1603) offered substantial support after 1585 to the Dutch rebels against Philip II and after 1589 to the Protestant Henry IV of France against his more powerful Catholic subjects; Philip II of Spain (1556–98), for his part, sent troops and treasure to the French Catholics, while his son Philip III (1598–1621) did the same for the German Catholics.

This willingness to assist arose because every court in Europe believed in a sort of domino theory, which argued that, if one side won a local war, the rest of Europe would inevitably be affected. The Spanish version of the theory was expressed in a letter from Archduchess Isabella, regent of the Spanish Netherlands, to her master Philip IV in 1623: “It would not be in the interests of Your Majesty to allow the Emperor or the Catholic cause to go down, because of the harm it would do to the possessions of Your Majesty in the Netherlands and Italy.” Thus, the religious tensions released by the Reformation eventually pitted two incompatible ideologies against each other; this in turn initiated civil wars that lasted 30 years (in the case of France and Germany) and even 80 years (in the Netherlands), largely because all the courts of Europe saw that the outcome of each confrontation would affect the balance of power for a decade, a generation, perhaps forever.