I wonder which battles which happened in our history would rival the epic splendor of the Space Marines. Here are some candidates for soldiers which could have been considered for Astartes training in real life.
1/ Spartans at Thermopolae. The story of how 300 Spartans and a few thousand greek hoplites held back a huge army of Persians. That is pretty much a real live version of a small group of armored warriors holding back a horde. Despite the fantasy feel of the movie 300, the history does say that a smaller force of Greek hoplites held back an army of Persians which could have numbered about quarter of a million Persians.
2/ The Army of Alexander the Greek. Alexander, and his armored legions were able to conquer the Persian empire despite having about 100 times the size and over 20 times the population.
3/ The Conquistadores, my personal favorite. Cortez and a force of probaby 150 conqistadores manage to conquer the Aztec empire which had armies numbering into the tens of thousands.
4/ The defenders of the Kibbutz. I read about a battle in Israel in 1948 where a bunch of isrelis, numbering no more than 15 or 30, managed to hold back an entire brigade of Egyptian soldiers. I am not sure if that is due to the exceptional fighting skills of the Israeli army or the lousy fighting skills of the Egyptians.
Any other suggestions?