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– When German officials headed to the NATO summit in Brussels this week, they were already prepared for what they considered to be an inevitable attack by President Donald Trump over their low military spending.

But on Wednesday morning, Trump took aim at the Germans for a very different reason: an 800-mile-long planned pipeline beneath the Baltic Sea. The German government has been pursuing its Nord Stream 2 project for years, despite criticism from the United States and some Eastern European nations.

Trump renewed the long-standing U.S. criticism of the project on Wednesday, and doubled down by tying it to the future of NATO. "Germany, as far as I'm concerned, is captive to Russia because it's getting so much of its energy from Russia," Trump told NATO Secretary General Jens Stoltenberg, speaking on camera. "We have to talk about the billions and billions of dollars that's being paid to the country we're supposed to be protecting you against."

Germany is indeed Russia's biggest export market in Europe for gas, with a dependency that may grow once Nord Stream 2 is finished. The project would roughly double Russia's export volume via the Baltic route that goes through the original Nord Stream pipeline.

Over the next few decades, Europe's own gas resources — which accounted for about a third of its supplies in 2016 — are expected to gradually disappear. (Britain, Norway and the Netherlands are Western and Northern Europe's biggest producers, primarily relying on natural gas fields in the North Sea.)

As Europe's own supplies are running out, the United States is hoping to gain access to a profitable market with growing demand.

But U.S. economic interests only partially explain why the pipeline conflict is now emerging as a point of contention.

Nations such as Poland and Ukraine also fear that Russia may be diversifying its gas routes into Europe to be able to exploit its grid for political reasons. In June 2014, amid the fallout over the Russian annexation of Crimea months earlier, Russia cut off Ukraine's gas supplies for weeks in what Kiev said was an attempt to blackmail Ukraine. E.U. pressure on Russia helped to eventually solve the conflict, as powerful member states in Western Europe grew concerned that the supply disruption might have ripple effects across the continent.

Ukraine and parts of Eastern Europe fear their partners to the west may be much less vocal next time if they receive their natural gas through a different set of pipelines, allowing Russia to cut off its unruly neighbors with impunity.

German Chancellor Angela Merkel has not shown any willingness to halt the controversial pipeline project, but at times she has indicated at least some skepticism, acknowledging that the project was not an entirely economic one but also of political significance.

Germany's Cold War legacy may help explain why Berlin is so vehement on making itself more dependent on a nation that has annexed foreign territories and cut off energy supplies in the past.

So far, however, Nord Stream 2 has only had one real impact: driving a wedge between Germany and other Western nations.

In response to Trump's accusations that Germany was captive to Russia, Merkel — who grew up in East Germany — on Wednesday cautioned the president that she may be in a better position to judge her country's dependence. "I've experienced myself a part of Germany controlled by the Soviet Union, and I'm very happy today that we are united in freedom," Merkel said.