Europe’s unity is cracking at its eastern edge, where a relationship once central to the EU’s postwar stability is souring. Behind the polite smiles of Donald Tusk and Friedrich Merz lies a souring relationship that could yet shape the future of the Union.

It’s true that the two leaders do get on quite well on a personal level. Donald Tusk, Poland’s premier, is probably the most pro-German politician you can find in Poland. Meanwhile, there hasn’t been a more pro-Polish German chancellor in recent times than Friedrich Merz. The two are members of the same party in the European Parliament, and yet they both keep messing it up.

Tusk has a reputation for making serial misjudgements. In the UK, he is best known for his unhelpful interventions in the Brexit referendum while president of the European Council. Misunderstanding Britain’s internal political dynamics when making the case for Europe back in 2016, he ended up driving doubting Remain voters into the Brexit camp.

Last week, we had another one of those Tusk blunders — in the form of a tweet about Nord Stream 2, the Baltic Sea gas pipelines which were blown up by Ukrainian commandos in 2021. Responding to Germany’s request to extradite one of the men allegedly involved, Tusk wrote: “The problem with Nord Stream 2 is not that it was blown up. The problem is that it was built.”

In one respect he is right. Poland has long argued that the pipelines made Europe too dependent on Russian energy. Germany massively damaged Europe’s security interests by seeking private commercial deals with Russia back then, even after Vladimir Putin annexed Crimea.

But to condone such violent destruction is an act of political madness. Imagine someone had said: “The problem is not that President X got assassinated. The problem is that he or she was elected in the first place.” You would presumably be appalled, regardless of your political position in respect to the president in question. Mercifully, nobody was killed when the explosions ended the gas supplies to Germany overnight. And luckily the sabotage didn’t leave Germany without power — it was a mild winter and the government did a good job managing the transition to new suppliers.

But the destruction was unequivocally a criminal act. The pipelines had been legally constructed and commissioned. They were private property. And we now know, thanks to the German prosecutor, that a group of operatives with links to the Ukrainian military and secret service are suspected of carrying out the sabotage. It was, without a doubt, the military operation of a state. From Tusk’s comment, then, you would think that Poland now officially condones an act of terrorism.

But Tusk wasn’t the first Polish politician to support the violent destruction of the pipelines. Radoslaw Sikorski, Poland’s current foreign minister, was an MEP at the time of the explosion, and he tweeted: “Thank you, USA”, in what turned out to be a mistaken reference to the US as the force behind the explosions. Given such comments, and given what we now know about the explosions, it is politically impossible for Germany to adopt pro-Polish positions.

The Germans, though, are as much at fault when it comes to provoking its neighbour. Despite his emollient diplomacy towards Poland, Merz’s first political act after being elected chancellor in May was to erect border controls to stem the flow of illegal immigrants. Acting unilaterally, the premier ditched the rules of the Schengen passport-free travel zone by invoking emergency clauses over immigration, causing chaos in Poland. His reasons for doing so were barely credible, and some would say even risked breaking EU asylum law. After all, the last big wave of immigrants came from Ukraine in 2022, after Russia’s invasion. The numbers had stabilised. For now, Poland and Germany both have one million Ukrainian immigrants. But this is a far bigger load for Poland, which has only half the population, and a fifth of Germany’s economic output. To close the German-Polish border to stem the flow of immigrants constitutes a naked act of beggar-thy-neighbour politics.

But, then, this is the same mindset which caused Nord Stream to be built in the first place — it’s Merkel’s mindset. She and Gerhard Schröder both masterminded the deal and after leaving office, Schröder even became the head of the consortium that built it.

Merkel, meanwhile, might no longer be politically active, but she has reappeared on the scene to defend her record and stoke internal discord within the EU. In an interview with the Hungarian media company Partizán last week she inflamed the Poles by blaming them for Putin’s invasion of Ukraine. “In June 2021, I felt that the Minsk Agreement was no longer being taken seriously by Putin, and this is why I wanted a new format in which we, as the European Union, would speak directly with Putin,” she said. “This was not supported by some. These were mainly the Baltic states, but Poland was also against it.”

There were good reasons why Poland and the Baltic Republics did not support another Merkel-led diplomatic initiative. They saw it as another way for Germany to maintain its toxic relationship with Russia to the detriment of Eastern Europe. I thought at the time that Poland’s objections to Merkel’s suggestion were reasonable.

But virtually all of Merkel’s later foreign policy initiatives were failures. She will go down in history both as one of the architects of the Russian-German political romance, and as a leader who failed to arrest Germany’s still ongoing economic decline. I criticised her often over her refusal to resolve the eurozone’s crisis for reasons of political expediency. I see this failure as the quintessential cause behind the EU’s political weakness today.

These growing fractures in key relationships binding the EU together will have an unavoidable and significant effect on national politics. The current investigation into the Nord Stream sabotage is a case in point. As Poland deliberates over Germany’s extradition request for Volodymyr Z, one of the suspected Ukrainian perpetrators, Tusk’s comments appeared to exonerate him. This has enormous potential for blowing up in the faces of the German and Polish leaders, and it plays straight into the hands of the AfD, which is currently outpolling Merz’s CDU/CSU. For the Right-wing nationalists, deeply critical of Ukraine, it is a story that will keep on giving. The German court trial will reveal a lot of unpleasant details of the operation. There will be a lot of emphasis on the links between the perpetrators and the powers that were behind it. I don’t for a moment believe the story that this was a privately funded operation. This trial could have a profound impact on German public opinion.

As things escalate, we will inevitably see the rise of bilateral tensions over Ukraine’s EU accession. Germany’s current coalition supports it, but on one condition: the EU budget has to be reformed first. Germany does not want to have to bankroll the reconstruction of Ukraine by itself. If the Germans get their way, this would invariably be at Poland’s expense. Right now, Poland is the biggest net recipient of the EU budget, and Germany the biggest net contributor.

“As things escalate, we will inevitably see the rise of bilateral tensions over Ukraine’s EU accession.”

For the EU to continue to function as a serious geopolitical force, then, a repair in the German-Polish relationship is a minimal requirement. But the way both sides are behaving at the moment suggests this is not going to happen. If they remain unresolved, though, these current political squabbles could prove explosive.