Defense Secretary Jim Mattis. (Virginia Mayo/Pool/AP)
Contributing columnist

Europe woke to the new realities of an unbound Donald Trump in the White House. The days of “the adults” being around are clearly over. True Twitter Trump has descended on the world.

The withdrawal from Syria is generally seen as unfortunately hasty. The consequences of the troop reduction in Afghanistan are seen as potentially dangerous.

Few documents will be read with as much concern as the resignation letter of Defense Secretary Jim Mattis released on Thursday. It was full of implicit warnings of what might lie ahead. European allies and friends will note his words on “treating allies with respect” and being “clear-eyed about both malign actors and strategic competitors” and will read with outright alarm that there might come a successor “whose views are better aligned” with those of the president on these and other subjects.

It couldn’t have sounded worse.

The past two years have been rich with tensions across the Atlantic. Trade threats, tariffs motivated by national security, the United States walking out of the Paris climate agreement and attempts to tear up the Iran nuclear agreement have been the most explicit points of contention. Not to mention the series of near-disasters of bilateral and multilateral meetings.

But for the Europeans, there has always been a bulwark of solidarity against the rage of the Oval Office on the other side of the Potomac. The Pentagon has not only kept the bonds across the Atlantic strong in these years of chaos, but in important respects, it has also reinforced them.

Initially, there were fears in Europe that the new Trump administration would not honor President Barack Obama’s commitments to begin strengthening the U.S. contribution to the defense of Europe. Under Mattis, these efforts were strengthened even further as part of the European Reassurance Initiative.

As long as there was a policy divide across the Potomac, the policy divide across the Atlantic was more manageable.

But now? When it comes to dealing with allies as well with “malign actors and strategic competitors,” if a new defense secretary is more aligned with Trump’s tweet-storms, we could be entering a very different and very bumpy period. Even if Mattis attends the NATO defense ministerial summit in February, what happens thereafter?

It’s far from inconceivable that the usual rant about the need for the Europeans to increase defense spending will start to be accompanied with concrete threats. The Germans are certainly boosting defense spending, but they will not be able to move rapidly to any 2 percent target, and in addition, German cars will continue to be visible on the streets of New York.

Top Trump economic adviser Gary Cohen (who is now since long gone) was able to hold back Trump instincts to immediately slash tariffs on everything that looked German or European. And Mattis was able to retain the faith of allies in the key Article 5 transatlantic security commitment.

Now Europe is left with Secretary of State Mike Pompeo, who looks more obsessed with a hyped-up threat from Iran than practically anything else, and who spent his recent time in Brussels bluntly saying that most things the Europeans believe in and have been working for are plain wrong in Trump’s point of view.

It’s a strange new world. The Europeans certainly have their own share of challenges to deal with. That they might lose the last reliable anchor across the Atlantic will not make it easier.

Read more:

The Post’s View: With Mattis leaving, be afraid

Jennifer Rubin: With Mattis out, we’re in uncharted territory

Henry Olsen: Trump is now forging foreign policy on his own. Where will he take us?

Greg Sargent: As Trump rages over his wall, Mattis’s resignation hints at an alarming truth