After securing a foothold on the European continent, by late July 1944 the Allied advance had stalled. Mired down in Normandy, the Allies faced stiff German resistance and terrain which greatly favored the defender. Pushing further into the continent seemed unlikely.
To break the stalemate, Allied planners created a plan called "Operation Cobra," first bombing German defenses and then punch through German lines to gain maneuvering room for mechanized forces, and ultimately advance further into the European continent. The plan entailed a massive air bombardment of German defenders over an area of approximately 5 sq. miles. The offensive began on 25 July. In less less than 45 minutes Allied bombers dropped over 4,700 tons of explosives. This supported the attack of several infantry divisions which created a breach in the German lines through which armored units could then exploit. In terms that would become popular to describe these actions, the bombing created an opportunity for the Allies to "breakout" of the stalemate they had faced, and allowed them to "pursue" the Germans into the balance of Europe, eventually overcoming the Axis forces. Operation Cobra lasted from 25-31 Jul 44.
Image of LTG Omar Bradley and General George C Marshall, U. S. Army Chief of Staff, on the ground in Normandy from the Army.mil: http://www.army.mil/article/42658/_quot_Operation_COBRA_and_the_Breakout_at_Normandy__quot_/
One of the most prominent features of the Normandy region was the bocage country. The bocage was an irregular lattice of hedgerows that covered most of the peninsula at the time. Dating back to the Normans, these hedgerows were constructed to mark farming boundaries and enclose animals. The bocage presented a great obstacle to the advancing Allies and greatly favored the defending Germans.
Though the Allies had trained to fight in a variety of conditions, they were unprepared for this terrain and the unique challenges it presented.
Image of Operation Cobra map from USMA History Depart Atlas collection: http://www.westpoint.edu/history/SiteAssets/SitePages/World%20War%20II%20Europe/WWIIEurope62.gif