What were the MAIN differences in the war in the Pacific to the war in Europe?
By this I mean... Was their command structure different? What about the Terrain? The enemies that they were fighting?
There were many important differences between the war in Europe and the war in the Pacific. Obviously, the first difference was in the terrain. The war in Europe was fought primarily on land. Yes, there was submarine warfare and naval blockades and crossing of rivers and the Mediterranean Sea and of the Atlantic Ocean. But the war in the Pacific was dominated by the huge expanses of water that had to be crossed, on the water or by air, in order to engage the enemy.
Weather was another variable that affected the two theaters differently. Over the stretch of the war, those fighting in different areas of Europe experienced weather ranging from summer in the Sahara Desert to winter outside Moscow or St. Petersburg. The conditions challenged men and equipment, both directly and in terms of medical complications caused by cold, heat, precipitation of all kinds, etc. In the Pacific, the weather caused different kinds of challenges as fighting took place under tropical heat and humidity with the presence of health complications related to that part of the world.
There were differences in command structure for sure. In Europe, the US and the British had to share command with, for example, Eisenhower and Montgomery disagreeing with one another quite regularly. By contrast, the Pacific was completely an American show. There was no real need to coordinate with Allies in that theater.
Another major difference was the sort of weapons that could be brought to bear. The European theater was influenced strongly by tank warfare on the plains of Europe. By contrast, the Pacific theater offerred very little in the way of open spaces. Fighting in the Pacific was much more reliant on infantry and was done in jungle to a large extent.
In addition, the enemy was very different. The Germans were much more of a traditional enemy who would do things like surrendering. By contrast, the Japanese defended so unrelentingly that very few Japanese defenders survived battles like those on Saipan and Iwo Jima. This made for a very different fighting experience.