Germany's Angela Merkel has clearly had it with President Donald Trump, saying this weekend that Europe must fight for its own "future and destiny" after several days of contentious meetings with her American counterpart.

Good.

President Trump has already shot back, decrying Germany's trade surplus with the U.S. and its relatively small defense budget.

Also good. And there are a lot of reasons why.

First off, let's admit that Merkel's got a point. But it's not for the reasons she'd like the world to believe. President Trump and his take on global priorities are indeed a major threat, but to the European political class and not to Europe itself. Because the Trump message runs in direct opposition to two key narratives Europe's current top leaders need, or at least they think they need, to stay in power.

The first key contrast is on the issue of defense. President Trump's insistent and repeated call for Europe to spend more on its own defense and in contributions to NATO caused the most immediate uproar during the NATO and G-7 summit meetings.

And you can see why. The biggest challenge to the established political leaders in Europe is defense-related. The spate of terrorist attacks and other disturbing incidents inspired by radical Islamism have left more and more voters disenchanted with their level of domestic safety.

This is all heavily connected to the E.U.'s open border policies and the overall war on ISIS that the E.U. is widely , (but not completely), sitting out. It was that disenchantment that pushed the Brexit vote last year and gave Marine Le Pen, a formerly fringe candidate at the head of a fringe party, a stunning second place finish in France's presidential election earlier this month.

The numbers don't lie. While the U.S. spends more than 3 percent of its massive GDP on defense, Germany barely spends more than 1 percent according to the most recent data from the World Bank. France spends just about 2 percent, and the U.K.'s defense spending is under 2 percent of its GDP. And when you consider the fact that the U.S. has a much larger economy than any of those three European powers, the difference in real dollar amounts is huge.

No one likes to be publicly called a cheap skate, but Europe's defense austerity has political roots that stretch way beyond and deeper than the war on terror. The fact is that Western Europe's extensive and elaborate welfare state relies heavily on not having to foot major defense costs.

Whether its socialized medicine, hefty unemployment stipends, or guaranteed housing, the European budget simply can't afford to pay for all of those things and boost its defense spending at the same time.

As it is, even without the defense spending boost President Trump wants, that welfare state is already starting to collapse. Britain's National Health Service, just as one example, is floating the idea of asking patients to pony up extra cash to see a general practitioner. European governments have suddenly woken up to the fact that they need to literally breed more native born taxpaying productive citizens to cover their domestic costs. There's even a push in some countries to give married couples more time off so they procreate.

Now President Trump comes along and starts talking about defense. And it comes at the worst time as more and more Europeans are calling for better protection. In other words, the Trump push is shining a light on just how much Europe has been robbing Peter to pay Paul. And Merkel et al hate it.

But defense is just one part of this two-head Trump-induced monster. Any day now, the White House is expected to say the United States is backing out of the Paris climate deal. Just like President Trump's call for more European defense spending, this move pushes back hard on some of the European political class' most dominant financial obligations and political narratives of the last 25 years.

There are several big reasons why Europe's politicians, and career bureaucrats from all over the developed world, love to focus on climate change. First and foremost, it's one of those problems that's hard to define. They even changed the name of the problem from "global warming" to "climate change" to further muddy those waters.

In other words, any politician claiming that he's made a dent in the climate change problem is pretty hard to refute while any politician claiming her opponent has failed to make real climate change progress is similarly hard to debunk.

Meanwhile, a growing number of career bureaucrats get to keep on "working" for environmental improvements with almost no accountability. Compared to the war on terror, which does demand more tangible results, this is a much more politically pliable issue. For the political class, this is a win/win in perpetuity.

That is, until people like President Trump come around and try to slap some sense into the climate alarmists who are still relying on big U.S. financial contributions to their cause. Given the current political climate in Europe, this is more than an unwelcome interruption of a political narrative. In fact, it's more like a potentially cutting and pithy insult. As former Arkansas Governor Mike Huckabee said in President Trump's defense this weekend, "a beheading is worse than a sunburn." That kind of quip can really hurt.

There are some who believe that Emmanuel Macron's clear win over Le Pen in the French election and Angela Merkel's recent boost in the polls mean an end to the populist/nationalist streak in Europe that was more in-line with President Trump's messaging. But the Manchester bombing attack and every attack that will inevitably follow it threatens to reinvigorate the movement. Those attacks and the related migrant crisis have put the key job of any government -- protecting its citizens -- into embarrassing focus for all of Europe's political leaders.

Historical and diplomatic experts are correct when they say that America's commitment and backing of Europe since World War II are the key reasons why the continent never fell economically and politically into chaos. But the cost of that backing has begun to spike because of the European Union's border policies and the massive cost inflation for Europe's domestic welfare state.

President Trump is the first U.S. president since World War II who is willing to tell Europe that America can't keep backing it up even as Europe adds to its costs by refusing to budge on its open border policies and its expensive and economically damaging focus on climate change.

This is the only responsible course of action for any American president presiding over our own nation's spiraling welfare state costs and changing public priorities. Trump or no Trump, the U.S. cannot continue to back Europe unless Europe gets its own priorities straight and faces reality.

For the reality deniers that include Merkel, Macron, and even Britain's Theresa May, President Trump is indeed a nightmare. But for the European people, he may be their last best hope.

Commentary by Jake Novak, CNBC.com senior columnist. Follow him on Twitter @jakejakeny.

For more insight from CNBC contributors, follow @CNBCopinion on Twitter.

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