1964's Zulu, is certainly one of the best action films ever produced. Based upon "The Battle of Rork's Drift" in 1879. It's immediate precursor in history was, Zulu Dawn, released in 1979; which depicts the worst defeat in history, of the British Army, against a native force. The British were essentially annihilated by a much larger Zulu force.
Both films were written by Cy Endfield, and Zulu was also directed by him. Both films are generally accurate; but there are also some fictional scenes, characters and other directorial liberties.
These two films were the closet thing British filmmakers ever came to a "John Ford Western"...think "Fort Apache" 1948.