Of course, the political metaphor was clear, if you were connecting the story with the year that the movie was produced, .It was 1938, when Hitler became a new threat to Soviet Union. The movie wanted to warn the German, as well as to encourage the Russian, reminding both of them what had happened in history, sending a  warning message to the German" If you invade, We Russian will fight against you, just as our ancestors did in the 13 century", as well as an encouraging message to the Russian" If the Germans invade our motherland again, we should rise up and fight,just like our ancestors did in 13 century"

But after the signing of the soviet -german non aggression pact, the movie was banned, The Russian government stated that the movie might offend German, which was now a " friendly" nation to Russian

But was it the real reason? I do not think so, because the movie had another far hidden political metaphor, that would offend Stalin, but not German.

What was it.You might notice that before joining the anti German campaign, Prince Alexander was in exile, although he had a high military prestige. Only  after the Russian soldiers had been  crushed by The Germans, the political leaders asked Prince Alexander to return to the army,defending the nation

And this story plot had a hidden political metaphor, at this point , Prince Alexander was parallel to Leon Trotsky

, who also had a high military prestige, and who was also in exile( Mexico), when the movie was produced. The message was clear, Under the military threat of Hitler Germany, the Russian leaders(Especially Stalin) shall ask Trotsky to return from his exile, leading the Red army, fighting against Hitler, defending the motherland, 

Of course, Stalin did not like this idea, so he banned the movie, actually he even assassinated Trotsky in 1940, he really feared that other leaders might invite Trotsky to come back to Russia, and leading the Red army in the coming war against Germans

And this was the real reason to explain why this movie was banned