Is A like C? And is B like C? For virtually any A, B and C you come up with it is likely to be the case that the answers to these two questions are that in some respects A is like C and in other respects it isn't, and the same with B and C. If I had time to ponder it further, I might be prepared to say that this is true for every, and not just virtually any, A, B and C. But I'll stick with the weaker statement because it's all I need.
Now, let's call C fascism or even Nazism. In today's Guardian Albert Scardino seems more comfortable with the parallels to be drawn between the Bush administration (B) and mid-twentieth century fascism, than he is with those to be drawn between contemporary Islamist extremism (A) - 'Islamofascism' so-called - and mid-twentieth century fascism
This tells you one of the main things you need to know about the political ailments currently afflicting many liberals and leftists. They could do worse than to read Trotsky's writings of the early 1930s. Although Trotsky himself is unlikely to have accepted either of the above parallels, he cut through the kind of sorry and dangerous claptrap that minimizes or altogether eliminates the differences between democratic polities and the openly undemocratic, and murderous, movements that are intent on destroying them.