At the beginning of July Nato’s leaders will invite a handful of central European countries to negotiate for membership of the alliance. They hope thereby to disprove Thucydides’s first law of international relations: “They that have odds of power exact as much as they can, and the weak yield to such conditions as they can get.”

Over the centuries one great power after another has threatened the stability of Europe. The threat from France ended in 1815, that from Germany in 1945. Both times the victors were intelligent and self-interested enough to bring the defeated as equals into the comity of European nations. France and Germany are now (usually) co-operative members of western Europe’s most solid institutions, Nato and the EU. Neither is likely to threaten the continent again.

The threat from the Soviet Union ended in 1991. But we have not yet worked out how to bring Russia into a new European settlement. Russia is too large to join Nato and the EU, and it is too powerful to be ignored. The Russians believe that they are entitled to a say in the affairs of the continent to which they belong. The east Europeans hope that membership of Nato and the EU will insure them against their neighbour to the east. Both sets of aspirations are understandable. But they are difficult to reconcile.

Most countries would prefer to be on the giving rather than the receiving end of Thucydides’s law. In eastern Europe many have tried their luck when they thought history was going their way. Poland’s repeated attempts to dominate Ukraine started in the 16th century and finally petered out in the early 1950s. Austria, Prussia and Russia divided up Poland three times in the 18th century. Thereafter Russian troops imposed “peace” upon Warsaw on at least five occasions. That is to say nothing of the innumerable wars in the Balkans, provoked by ethnic strife and the meddling of outside powers. And even the Russians have seen their national existence threatened by P…

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