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To stay on topic, why do you think the Ukrainian Ambassador to Germany Melnyk had to go. He was not diplomatic in the best sense but openly clear and plain, as is fitting when you are a country to be wiped out by an evil aggressor. Perhaps he knew too much about the German government and its venality or blackmail by the Russians. Sorry it's just my guesses and it's my common sense. I may be wrong.
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On the surface of it Melnyk had to go because he visited the grave of Stepan Bandera, a rather controversial figure of Ukrainian nationalism before and during the Second World War. This was not viewed in a good light by the German government.
The Ukrainian history in the last few hundred years is a complex story of domination by the Russian Empire and Poland, followed by the Soviet Union. Bandera did side with the Nazis in an effort to gain Ukrainian independence and was also anti-polish (understandably) and anti-semitic but there is no evidence that he approved of, or even was aware of, the massacres that took place in Ukraine at that time. In fact he was in a German concentration camp at the time. He has been demonised, largely by Russian propaganda, as a Nazi but honoured by most Ukrainians as a fighter for independence. In the South and Southeast of Ukraine, which has a greater population of Russian speakers, Bandera is more likely to be viewed as a Nazi. For the Russians, and Putin's propaganda machine, Ukrainian nationalism is associated with Bandera, and therefore Nazism. Given the whipped up racist anti-ukranian sentiments in Russia and desire for domination and annihilation it is easy to see where the Nazi label belongs.
Worth noting that Bandera was one of so many anti-soviets assassinated by the KGB, in his case by a cyanide spray. The means used now are even more deadly and dangerous but the practice continues. (I wonder just how many anti-Putin, or "inconvenient", people have been assassinated since Putin came to power.)
It is worth reading
https://en.m.wikipedia.org/wiki/Stepan_Bandera