The extraordinary pair of meetings in the past week — the Trump-Putin summit in Anchorage, Alaska, on Friday and Trump’s hosting of the leaders of Ukraine, Germany, France, Britain, Italy, Finland, as well as the NATO alliance and the European Commission — were prompted by the latest iteration of a continuing source of instability over hundreds of years.

That instability is Russia‘s westward expansion across the great northern European plains, bereft of natural boundaries, from the principality of Muscovy in the early 1500s to the Soviet empire and satellites ruled by Stalin and his predecessors for nearly half a century after 1945.