(Publisher: This document (PDF) is a Canadian Document released to the Public,... but has USofA all over it)
Urban terrain possesses all of the characteristics of the natural landscape, coupled with manmade construction. Countless rooms, hallways, stairwells, streets, alleys, subterranean corridors, rooftops, and internal space make the task of controlling the urban area near impossible.
Considerations unique to the urban environment include the lack of ability to blend in with the terrain and population, decreased operating areas due to the vertical structures and subterranean systems, and degraded communications due to “urban canyons” [Sumner, 2001].
Consequently, urban areas deprive well-equipped armies (high-tech forces) of many of the technical advantages that were developed during the Cold War. Urban areas constrain maneuver, strain C3I systems, and raise substantial political problems by putting noncombatants and non-military targets in the way of military forces [Peters, 1997a][Hahn and Jezior, 1999].
As a result, even a 20:1 exchange rate might not be a big enough advantage for U.S. Forces [Press 1999].
This ratio is quantified to a range between 9:1 and 27:1 i.e. 27 attackers per defender by [U.S. Army, 2000].
The previous ratios can encourage adversary - regular troops, criminal gangs, vigilantes and paramilitary factions - to bring conflicts within urbanized areas, then removing their disadvantage against trained regular forces.