Newsrooms and foreign ministries around the world are full of quotations, many of which have become detached from their original authors. One of the most frequently cited is attributed to Lord Palmerston, the 19th-century British prime minister. Addressing the House of Commons in 1848, he articulated a principle that remains central to diplomacy: Britain had no eternal allies and no perpetual enemies, only interests that were eternal and perpetual. It was a view of foreign policy that placed national interest above sentiment. Nearly two centuries later, his words remain among the closest things diplomacy has to a law of physics.

Palmerston was not simply being cynical. He was describing something every state, large or small, eventually learns: friendship between nations is not like friendship between people. It does not endure on memory or sentiment alone. It lasts as long as it serves a purpose. When that purpose changes, the relationship changes with it. No formal letter is necessarily required to announce the shift. It becomes visible in trade figures, troop movements and, sometimes, in who picks up the phone first after a crisis.

Geography makes this reality difficult to escape. In Prisoners of Geography, Tim Marshall argues that a country's mountains, rivers, coastlines and neighbours constrain its choices long before its leaders make them. Russia's exposed western approaches help explain its longstanding concern with strategic depth and buffer zones. Saudi Arabia's position between the Gulf and the Red Sea, amid a region of competing interests and shifting partnerships, helps explain why Riyadh has sought to diversify its security relationships. Geography does not care about loyalty. It concerns itself with........

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