It's as if Matthias Müller, the CEO of German car giant Volkswagen, thought he was talking to one of his own executives when he wrote to European Commission President Jean-Claude Juncker in March to object to the company's treatment over the Dieselgate affair.

Müller complained about one of Juncker's commissioners and asked the boss to have a word with her and tighten the leash. He expressed hope this would bring an end to a story that has caused him considerable headaches.

Since the affair broke in the fall of 2015, the question of whether Volkswagen will compensate millions of affected European customers has been a source of frustration to the commissioners involved — in particular, Vera Jourová, who oversees the justice and consumers portfolio. She doesn't have the power to force Volkswagen's hand, but that doesn't mean she hasn't been trying to put political pressure on the carmaker to offer compensation.

She successfully annoyed Müller, who expressed his irritation at the Commission's efforts in a letter to Juncker dated March 27, obtained by POLITICO. "In our view, it is not within the competence of the Commission to enforce European consumer rights," he wrote.

Müller might be the head of the EU's "biggest industrial corporation," as he wrote in his letter. But he made a major misjudgment about the way Juncker sees his role and how he runs his Commission.

Here are three clear lessons from an exchange of strongly worded letters.

1. When on the defensive, don't be aggressive.

In his letter, Müller said Brussels — specifically in the form of Jourová — was overstepping its competencies. He called for the Commission to stop pushing the issue, saying there was no legal basis for compensating consumers — and accused Brussels of being partisan: "I urge you to ensure that the Commission complies with its duty of being impartial," Müller wrote.

Juncker pointedly disagreed. "The Commission always ensures that it acts within its responsibilities," he wrote back in a letter to Müller on Friday, also obtained by POLITICO. He also cited a legal basis for the Commission's action (it's Regulation (EC) No 2006/2004, for those interested) on consumer protection cooperation.

While the Commission may have no executive powers in the matter, it is "facilitating this exchange of information" — as there's a lot at stake for Brussels, "particularly as it concerns a process involving more than 8.5 million consumers in the EU," Juncker wrote. That's the number of cars in Europe that were found to have included defeat devices, able to skirt emissions tests.

Juncker's message was: Do your homework. We're a political Commission and we can only win in this game.

2. Don't think I don't talk to my people.

In a February meeting with Jourová, Müller had rejected her call to compensate customers. Brussels didn't let up, however. In March, national consumer authorities, with the help from Jourová, started preparing a joint enforcement action.

In his letter, Müller wrote that "there is no legal basis for compensation," reiterating VW's commitment to fixing affected cars by fall this year.

"However, I have repeatedly found that the Commission is asking for compensation, although Commissioner Jourová and I agreed that compensation can not be considered," Müller wrote.

That was news to Juncker. "According to her own statement, Ms Jourová has encouraged you to offer a voluntary indemnity as a commercial gesture, while fully respecting the legal framework," he wrote back two weeks later — and after asking for Jourová's view, according to Commission sources.

"Such a step would show that your company is not indifferent to the inconvenience that has been caused for consumers since October 2015," he said. Offering voluntary compensation, as a "commercial gesture" to affected consumers could also "counteract efforts to lodge civil claims for damages in member states."

What Juncker meant: Don't expect me to be disloyal to my people. And don’t believe everything you read about my (or my lieutenant's) tough management style. Sure, I’m in charge, but if you attack one of my team, you attack me.

3. Don't bother me again. And don't complain in writing, stupid.

"Please do not hesitate to contact me for further information," Müller wrote.

Juncker won't.

"I would like to encourage you to continue your dialogue with Commissioner Jourová on all the above points, so that in the end a fair treatment of all consumers in the EU can be ensured," he replied.

Letters to the Commission president are archived, as are his answers — and they occasionally get leaked. How could anyone imagine that Juncker would reply in any other way than he did?

Juncker's message, at least implicitly, was: What were you thinking? Stop writing letters and call me if you want to talk.