After fall of 1914, World War I on the western front was characterized by trench warfare. That is to say that soldiers in trenches protected by machine guns and barbed wire occupied such strong defenses that attacks involving hundreds of thousands of men would do well to advance a few miles at a time. This lasted until the spring of 1918 when German numerical superiority and new, "storm" tactics upset the balance one way, and the arrival of fresh American troops and new equipment such as tanks upset the balance the other way.
Germany's eastern front was not nearly as static (against the Russians). Why was that? Was it because of factors such as weather and terrain, or were the Russians slower to adopt trench warfare tactics than the Germans, British, or French?