I believe this is the best John Wayne movie. Non-fans often say he can't act - let them watch this and they will change their mind! I read a quote of Wayne's on-line, where he said (paraphrasing), 'People used to say I play myself in my movies, well I didn't, I acted; I've got to the stage in my career where I just play myself when I want to' (One can see this thought more obviously in Rio Bravo, for instance. However, in 'The Searchers', Wayne puts on a scorching performance in his facial reaction to seeing some girls who have been left out in the wilds too long, whilst he is looking for his niece, Natalie Wood. This still can be seen in a book called the 60 Greatest Movies). The Horse Soldiers is a movie where Wayne acts in such a diverse and interesting way. For instance, his constant fights with William Holden (the doctor); his interactions with 'Miss Hannah Hunter'; his dealing with the southern soldiers who have deserted and held up the local Sherrif (?) at a barn below southern lines; his loss of temper (culminating in a kick up the arse) with Colonel Secor - appropriate, when one finds out that Wayne's character "used to build engines for a living, you know!" - after Secor boasts of the Union army destroying them at a rail junction town - ; and his explanation of his dislike for doctors to Hannah Hunter at Newton Station (as mentioned above) - Quote goes something like: "And I didn't kill him! I don't know why. Must have been drunk, or too conventional!
The story has adventure, proper cavalry etiquette to learn from (as a non-American), true (in a movie style sense) history with boys being called in to fight and friend vs. friend, humour, and great characters. Including the 'regular' Wayne friend, Hank Worden as 'The Deacon', with more of his classic quotes:e.g. Wayne: Deacon, how far to the river? (Trying to out run southern soldiers).
Deacon (Hank Warden: "I've been baptized, Colonel; I've been baptized!"
The most important factor is an attempt to characterize the brutality of this - or any) civil war - the amputation scene being a standout.