How did war progress on the western and eastern fronts?

While World War I on the Western Front developed into trench warfare, the battle lines on the Eastern Front were much more fluid and trenches never truly developed. This was because the greater length of the front ensured that the density of soldiers in the line was lower so the line was easier to break.

Primary Source

Explanation:

The picture was taken at the actual time and event in history so it is a primary source.