This sentence is aimed at someone who does not know what the Nuremberg Trials are, and indeed that there was a series of military tribunals. That's why there is an indefinite article. If in the same situation a definite article were used, and the person who hears the sentence did not know that such trials existed, he would reply "what trials are you talking about?". A definite article supposes knowledge. It would mean that this specific series of tribunals had been mentioned before or that the listener knows that this specific series existed. In general, concepts which have not been introduced or mentioned yet are introduced using the indefinite article.

You can't drop the article altogether because "series" is a singular noun, even if it doesn't look like one.