‘Divide and rule’ (or ‘conquer’) diplomacy aims to disunite the opposition, the better to control it. The ancients were masters of it. So is the EU, unlike the UK.

In the 4th c bc Philip II of Macedon played the game very skilfully as he plotted his conquest of Greece. Taking full advantage of the fact that the Greek city-states spent most of their time quarrelling with each other, he offered the hand of friendship to some, used his powerful army against others, and all the time offered peace terms to Athens.