He's a lot of things, but not an idiot. Varoufakis was one of the few finance ministers who is actually an economist (as opposed to e.g. a jurist), so he was very qualified. But he went into the negotiations thinking that everybody was trying to solve the problem amicably, that they were trying to find a compromize everybody could live with. But no, it was pure power play. Germany and co wanted to keep the status quo of a Greece close to bankrupt that they could boss around. And I think they wanted the left-wing "experiment" to fail under all circumstances, to set an example.

I think that's part of the problem, Varoufakis is intelligent and an economist, yes, but really not a diplomat, and his job here as Greece's main negotiator was more a diplomat's than an economist's job. He was very naive, as his book somewhat admits, about how EU politics work (and to some extent about Greek politics as well, where he assumed Tsipras was on the same page as he was).

Stathis Kouvelakis, a member of Syriza's Left Platform faction that later split from the party, had this criticism [1] about both Varoufakis and his successor Tsakalotos (another economics professor), which I think has some truth to it:

Tsakalotos said he was very disappointed by the low level of the discussion. In the interview to the New Statesman, Varoufakis says very similar things about his own experience, although his style is clearly more confrontational than Tsakalotos's. From this it is quite clear that these people were expecting the confrontation with the EU to happen along the lines of an academic conference when you go with a nice paper and you expect a kind of nice counter-paper to be presented. I think this is telling about what the Left is about today. The Left is filled with lots of people who are well-meaning, but who are totally impotent on the field of real politics.

[1] https://www.jacobinmag.com/2015/07/tsipras-varoufakis-kouvel...

Yes, they were really naive thinking that the meetings of the ministers of economy were about economics and how to improve the lives of the citizens.

This is telling about what the Euroarea is about today.

I suppose, at least it's a good thing if those naive professors, now that they are not so naive, explain the rest of citizens how the system really works.

His hand was quite full as long as Tsipras agreed to do what Varoufakis thought needed to be done. The moment Tsipras decided against him he lost his hand. So, his error was probably to trust Tsipras would do what needs to be done instead of clinging to power.