Take a look at this WW1 photo taken by Ernest Brooks.
Could it have been as powerful as a "normal" photograph? I doubt it very much. You don't need to know who the men are, just their purpose. It is a powerful symbol of the grim reality of war, men resigned to their duty.
To take a silhouette, simply choose a good shape for a subject, then photograph against the light exposing for the light background and not the subject. Use Aperture Priority for greater control and adjust the exposure as required until the desired result is achieved.