As if the Europeans needed another wake-up call about the contempt in which President Trump holds them, his mocking antipathy at the World Economic Forum in Davos, Switzerland, was a good reminder.

But Davos presented another lesson to Europe. Standing together on the principle of territorial integrity and sovereignty, while warning of severe economic countermeasures, the Europeans achieved an apparent retreat from Mr. Trump over Greenland.

Sovereignty and the inviolability of borders are fundamental tenets of the European project, built out of the ruins of World War II, when the aggressive imperialism of big powers led to millions of deaths. The lesson was clear: Defending borders collectively is the only way small states are protected from the predations of larger ones.

Now Europe finds itself again confronted by big powers with expansionist goals. Russia continues its effort to conquer Ukraine, whose sovereignty it had recognized in numerous treaties. And the United States has been demanding that Denmark, an E.U. and NATO ally, hand over Greenland.

But preserving territorial integrity and sovereignty is the red line, expressed both in the European Union, a collective of 27 nations, and in NATO, a military alliance of 32 nations. It can seem quixotic in the current world to be defending international law, the U.N. Charter and the Helsinki Accords, which all insist on the inviolability of borders, but in a sense, that is Europe’s fate.