Urban conflict is messy. It appears the Israelis are making at least an initial push into Gaza, and this is the terrain on which they are fighting. It is a textbook example of such combat, consisting almost exclusively of urban construction. Gaza is also densely populated and has a complex network of tunnels and other underground rooms and passages. The cost—to both Israel and Palestinians—of breaching such terrain could set off an unpredictable chain of regional events.
Urban terrain is a subset of what is referred to in military jargon as “close terrain”—a location in which the standoff advantages of a modern military are negated, in whole or part. Urban terrain is also the most challenging subset of close terrain as it is three-dimensional. Aside from someone who has climbed a tree, a Vietnam-like jungle, while definitely close terrain, is still a two-dimensional battlefield. But in a city, or a tunnel complex, it is very possible to be attacked from above and/or below.
A desert is more or less the polar opposite of close terrain. Here—as Saddam’s Iraqi Army found in 1991—airpower can destroy formations in depth, while the extended ranges of Western weapon systems—tanks, fighting vehicles, guided missiles—can engage with the enemy before the enemy can effectively return fire. Modern militaries are optimized for open terrain, and it’s where they prefer to fight.
Close terrain eliminates this advantage because you can’t know you are in danger until an enemy is inside your close range. Therefore the enemy is always—in the words of the Vietnamese—“hugging the belt,” or getting and staying close enough that the advantages of a modern military are (mostly) negated. Airpower and artillery superiority do you no good when the enemy is just a few yards away.