I guess the idea is that Alice wants to be 100% certain that Bob will attack before she commits her own forces, and Bob similarly wants to be 100% certain that Alice will attack. This level of certainty is not achievable.
Alice and Bob must each choose an actual policy for how many confirmations they need before they actually commit to attack. We can number the messages like this:
1. Alice's first message 2. Bob's first message 3. Alice's second message 4. Bob's second message ...
Alice's policy can only commit to attack after receiving some even-indexed message n. Bob can only commit to attack after receiving some odd-indexed message m. So n and m cannot possibly be equal. Therefore the possibility exists that a message fails between n and m, and either Alice or Bob ends up attacking without the other's support.