People tend to make and disseminate plans to their minions prior to executing them. Thus, the plan could be leaked before the thing that is planned occurs.
Easy solution: Make plans for every possible option, so you don't know which one will actually occur.
Oh, wait. The Pentagon already does this: The have battle/attack plans for every country in the world! So I guess they should leak those and let people scream in outrage?
A plan is not equal to intent. And you only know intent after it happens, not before.
"Here are the targets we'd attack in Canada if we were at war" is a little different from "everyone meet up at Fort Afghanistan in Tent 123 at 1300 hours".
Well, the thing of importance would be the enemy knowing a strike is coming (for example). If the military is having people come to a meeting, the subject of that meeting is already determined. A mole could release the details on Wikileaks, saying "they're meeting in Tent 123 to prepare for the attack on Kabul National Bank". Well, now the bank manager knows there's a strike force inbound.
Not that this is a problem inherent to Wikileaks; you could use any service to communicate the same thing. I'm just giving an example of a time where publishing sensitive documents could be done ahead of time to disastrous consequences.
