The movie shows the futility of the trench warfare in the Great War. When a French attack fails, the general decides to execute 100 soldiers as an example. He is then convinced by another general to reduce the number to three. The soldiers are tried in a trial without any evidence and then executed.
According to Wikipedia, the movie is based on a novel by Humphrey Cobb and loosely based on the true story of four French soldiers who were executed for mutiny in World War I but their executions were later ruled unfair. Also, the book and movie title comes from the following:
The boast of heraldry, the pomp of pow’r,
And all that beauty, all that wealth e’er gave,
Awaits alike th’inevitable hour.
The paths of glory lead but to the grave.
Overall, it is a great antiwar movie. I rate it 8/10.