[giantrobot]

One key to success in military operations is tempo. Armed defenders using guerrilla tactics disrupt an attacker's tempo. Every minute a platoon of soldiers hesitates crossing a street or gets bogged down clearing a building is a minute closer to sunset, a minute more of vehicles burning fuel, a minute longer for defenders to get reinforcements, and more stress and fatigue on the whole unit.

An attacking force doesn't have infinite resources to attack indefinitely. A mechanized force that runs out of fuel is extremely vulnerable. Those extra minutes burning gas not being able to push forward add up.