Im giving Sylvanas' army chance of getting into wetlands and not further. They will be spotted by scouts and spies and armed response will be given. The forsaken army is not built for war of attrition, nevermind they can raise humans to their cause. Most newly raised humans are straight out useless because they either become morons when they are raised, or they are so confused they can barely hold a blade, even if they were trained warriors in life.
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I tought you were talking about catapults. Not hundreds of B-17 flying fotresses.