The primary combat specifications theoretically allow for the case that the attacked place remains with 0 troops. In particular when 1 attacks 1 the probability should be 42% that this case happens since there is no attacking troop left to move in. Yet this case is ruled out by the statement that the defender's roll has no impact whether or not the territory is captured. So what is the exact rule?

Edited 5/10/2014 15:26:40

This is on the wiki somewhere, but I couldn't find it. Anyway here's how it works:

A atacc B
A have 1 army, B have 1 army.
A kill 1, B kill 1.
Now both have 0, since A can't take the territory B keeps it.
B now get an army, since he cannot have 0 armies there

So basicially, for a 1v1 to work, the attacker need to kill the 1 deffender and not die himself. This is primerly why doing 1v1 attacs usually is a bad idea. (same goes for 2v2 etc)

Edited 5/10/2014 15:37:32

Exactly how henns said. The attacker needs at least 1 troop left. When both armies zero out each other it's as Fizzer said the only situation, where the system "resurrects" one defending army. The territory remains in defender's hands.