The length of the front in the East was much longer than in the West. The theater of war was roughly delimited by the Baltic Sea in the West and Moscow in the East, a distance of 1,200 kilometers, and Saint Petersburg in the North and the Black Sea in the South, a distance of more than 1,600 kilometers. This had a drastic effect on the nature of the warfare. 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. Once broken, the sparse communication networks made it difficult for the defender to rush reinforcements to the rupture in the line to mount a rapid counteroffensive and seal off a breakthrough. There was also the fact that the terrain in the Eastern European theater was quite solid, often making it near impossible to construct anything resembling the complicated trench systems on the Western Front, which tended to have muddier and much more workable terrain. In short, on the Eastern front the side defending did not have the overwhelming advantages it had on the Western front. Because of this, front lines in the East kept on shifting throughout the conflict, and not just near the beginning and end of the fighting, as was the case in the West. In fact the greatest advance of the whole war was made in the East by the German Army in the summer of 1915. With the aid of numerous black and white and color photographs, many previously unpublished, the World War I series recreates the battles and campaigns that raged across the surface of the globe, on land, at sea and in the air. The text is complemented by full-color maps that guide the reader through specific actions and campaigns.