In 2008, then-presidential candidate Barack Obama's massive rally in Berlin reflected the desire of many Europeans to repair U.S. relations after divisions over the Iraq war. This week, President Obama, on his last official visit to Europe, again went to Berlin, and in a joint news conference with German Chancellor Angela Merkel reflected on the status of the transatlantic relationship. And later Obama, Merkel and other key European leaders agreed unanimously to keep sanctions on Russia for intervening in Ukraine.

President-elect Donald Trump, whose conciliatory and even flattering words regarding Russian President Vladimir Putin have caused anxiety among many European leaders, let alone Republicans like Arizona Sen. John McCain, should recognize that this type of Western cohesion is necessary in an unstable world.

Obama made that clear last week. "Our alliance with our NATO partners has been a cornerstone of U.S. foreign policy for nearly 70 years — in good times and in bad, and through presidents of both parties — because the United States has a fundamental interest in Europe's stability and security," he said. On Russia, Obama said that he hopes Trump "does not simply take a realpolitik approach."

For her part, Merkel said of Russia: "This is all about respecting certain principles. ... So the fact that for over 70 years we have been able to enjoy peace, to live in peace very much depends on territorial integrity and sovereignty of each and every European country being respected. In view of the European history, the reverse would be the start of a very bitter road down a slippery slope, and we have to nip this in the bud. We have to stand up resolutely against any such attempts."

The global instability the incoming president will face is best met through multinational efforts. Indeed, during the campaign Trump was reticent about America's overstretched global role. But building consensus requires cohesion, and Trump's intemperate remarks (Merkel is "ruining Germany," he tweeted after Merkel was tabbed Time's "Person of the Year") and his choice to meet Nigel Farage, the former United Kingdom Independence Party leader who championed Brexit, before U.K. Prime Minister Theresa May are the type of mistakes he must now avoid.

Both Obama and Merkel commented on the potent populism roiling both sides of the Atlantic, and in fact Brexit and Trump's triumph may be part of a broader pattern that could be repeated in upcoming European elections. But just as it would have been inappropriate for European leaders to get involved in the U.S. election, the next president should refrain from favoring anti-E.U. candidates. Indeed, Trump may soon learn the benefits, if not necessity, of strong transatlantic links.

Saying he was "cautiously optimistic about my successor and the shift from campaign mode to governance," Obama added that the presidency itself might be a tempering influence. "There's something about the solemn responsibility of that office, the extraordinary demands that are placed on the United States — not just by its own people but by people around the world — that forces you to focus, that demands seriousness."