The only reason it reaks political undertones is because freedom is a concept that is alive today.
The Persians did that to the Greek city states, the Greeks and Macedonians did the same, so did the Turks, the Mongols (which were made up of many different tribes, not just Mongols) the Muslim armies, the Crusaders, the British, the French, the Dutch, the Japanese, the Chinese.
No matter where you go your gonna hear about massive armies attacking natives 'fighting for freedom.'
Theres many stories like this in history. Two major ones (never had big movies made of them) being - the African Zulu (unless you count Shaka Zulu) defeat of a superior super power imperialist British & native force, and the Afghan British and Afghan Russian wars (unless you count Rambo III) that led to the defeat of two of the worlds super powers - Great Britain and the Soviet Union.
The only reason this movie is getting the light of day is because 1 - Frank Miller is a great writer and 2, Snyder had the vision to pull it off - no political undertones involved at all!