The Vanquished, by Brian Garfield
A fictionalized account of a real-life event. In the late 1850s, a group of about 100 American filibusters traveled to Mexico, contracted to help overthrow a corrupt district governor in exhange for land. But when they arrive, they discover the governor has been kicked out without their help and they are told to go home.
They opt not to go home. Violence ensues.
The primary point-of-view character is Charlie Evans, the youngest (15 years old) of the filibusters. Garfield, though, occasionally shifts the point-of-view to other characters, both North American and Mexican. This is a short novel, but Garfield still manages to tell the story well and give various characters three-dimensional personalities.
The action in the book is viceral and often brutal. Of particular note is a scene late in the book when the filibusters are besieged inside a mansion. The Mexicans are occupying a nearby church to keep them under fire. The description of a desperate attempt by the filibusters to attack and clear out the church is one of the most intense battle sequences I've run across in a long time.