David A. Andelman, editor emeritus of World Policy Journal and member of the board of contributors of USA Today, is the author of "A Shattered Peace: Versailles 1919 and the Price We Pay Today ." Follow him on Twitter @DavidAndelman . The views expressed in this commentary are his own.

(CNN) The red welcome mat is out at the White House. Angela Merkel in coming to town. This ought to be a cause for celebration. In terms of ideology, Merkel is perhaps the most conservative ruler of a major country in the European Union.

But look out, this lady is also as close to an Iron Chancellor as any German leader since Bismarck two centuries ago. She's tough, she has three terms under her belt and is headed for a fourth, and she has an agenda she's ready to unfold before her host, Donald Trump.

That agenda may not be in any sense contiguous with the one Trump has embraced, and that's where the friction could happen -- if it does. One can only hope it won't.

It's hard to say what's at the top of the agenda since there are so many points of friction before the two even meet: Russia, NATO, trade, climate change, refugees, counterterrorism. So, let's just take them in what should be their order.

Russia

No Western leader knows Vladimir Putin better than Angela Merkel. While she led Europe in pressing for tough sanctions against Russia for its seizure of Crimea and incursions into Eastern Ukraine, Putin respects her toughness and longevity in office.

Indeed, Merkel speaks to Putin virtually every week by telephone. She has told confidants that she sees it as an essential duty. Putin needs to hear from voices outside his immediate Kremlin circle. And without her regular contacts , he would not -- a potential catastrophe for the West.

If there is anyone who could teach Trump about Putin, it's Merkel. Hopefully, Trump will have the patience and good sense to listen as his course of instruction begins.

NATO

Merkel should be pleased that some of Trump's envoys have expressed a commitment to NATO somewhat more accepting than his sentiments during the campaign. Still, he continues to demand that NATO member nations increase to 2% of GDP their defense expenditures. Only four nations — Greece, Estonia, Britain and Poland — managed that level last year. Germany is number 14, with just 1.2% of its GDP. Still, that amounts to €37 billion ($39 billion) and is set to reach €39.2 billion by 2020.

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"We're moving in the right direction, but we can't do it in one year," German Defense Minister Ursula von der Leyen said in an interview with Reuters. But with a $54 billion increase in defense spending in the new Trump budget, and 3.6% of America's GDP already committed to defense — triple Germany's level — this could be a contentious issue.

Trump should listen to Merkel's quite sound reasoning that no major nation is an island and that, as her defense minister pointed out, "The most important thing is reliability."

Trade

These kinds of economics segue neatly into trade. Last year, Germany posted a $65 billion trade surplus with the United States and so far this year has been running ahead of last year. During the campaign, Trump had threatened to impose a 35% tariff on German carmakers who import their products into the United States. But Merkel could also point to the reality that in 2015, German direct investment in the United States hit $255 billion -- a lot of it German automakers' plants in the United States, employing 670,000 American workers producing cars here for American customers. US investment in Germany was less than half that level.

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Little is closer to the heart of Trump than trade talks, so here Merkel and Trump have a chance to reach some real understanding. While Trump has condemned multinational trade and tariff agreements like the Trans-Pacific Partnership, Merkel is still hoping to salvage the Trans-Atlantic Trade and Investment Partnership pact. With the European Union considering itself a single tariff unit, it might not be so hard for Merkel to persuade Trump that T-TIP is simply a nice, bilateral trade pact.

Climate change

Merkel has been one of the big backers of the Paris agreement to control greenhouse gases and work toward halting global warming. With the Environmental Protection Administration suffering the biggest cutbacks in the new Trump budget and the President lifting tough fuel economy standards from American automakers, it's hard to see how they could reach any sort of compromise in this meeting. But count on Merkel to make an opening argument in what will likely be a long and contentious issue.

Refugees and counterterrorism

If there is one issue that has been the most subject to the kinds of rhetoric calculated to poison relations between world leaders, it's refugees. During his campaign, Trump alternately praised Merkel as "a really great world leader," then promptly snapped that she was "ruining" her country by accepting hundreds of thousands of Syrian refugees, who Trump claims conceal hordes of terrorists.

He quickly pounced on the Christmas truck attack in Berlin by a Tunisian terrorist, swaggering "I've been proven to be right."

Now with Trump's latest immigration order suspended by a federal judge and Merkel facing her own internal attacks by opponents in her re-election bid, both sides would seem to need some mutual reinforcement, or at least a path toward lowering the rhetorical temperature.

Taking each other's measure

Above all, Merkel is here to take the measure of the successor to Barack Obama, with whom she had a remarkably chummy relationship. Clearly the stakes are high for this meeting: the future of the trans-Atlantic alliance, German leadership of Europe, a united front against Russian aggression, billions in trade and investment and a common front against terrorism.

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Yet it is hardly a meeting of equals. Merkel comes to this session utterly in command, at the very moment the electoral power of European Trump-style populism has been called into question with the sound defeat on Tuesday of the far-right candidate for Prime Minister in the Netherlands. By contrast, Trump is very much under attack on all sides, many of his core priorities still unrealized and still out of grasp. It is this dynamic, more than any other, that threatens a positive outcome.

Alternatively, Donald Trump could bask in the reflected popularity of Merkel, learning and profiting from her long years of all but unquestioned power and glory.

Still, there are limits. Merkel will not be going to Mar-a-Lago.