When Columbus triumphantly returned from America to Spain in 1493, his discoveries inflamed an already-smouldering conflict between Spain's renowned monarchs, Ferdinand and Isabella, and Portugal's João II. Which nation was to control the world's oceans? To quell the argument, Pope Alexander VI - the notorious Rodrigo Borgia - issued a proclamation laying the foundation for the Treaty of Tordesillas, an edict that created an imaginary line in the Atlantic Ocean dividing the entire known (and unknown) world between Spain and Portugal.

Just as the world's oceans were about to be opened by Columbus's epochal voyage, the treaty sought to limit the seas to these two favoured Catholic nations. The edict was to have a profound influence on world history: it propelled Spain and Portugal to superpower status, steered many other European nations on a collision course and became the central grievance in two centuries of international espionage, piracy and warfare.

At the heart of...