A and B must not attack alone, i.e. A must not attack if B is not going to attack, and B must not attack if A is not going to attack.
Now, message 1 (from A to B) says "attack!". So B knows that A wants B to attack.
...but A mustn't attack alone, so A won't attack unless A receives B's confirmation. So B sends message 2 ("sure!").
...but B mustn't attack unless B knows that A is going to attack, and A will only attack if A has received message 2. So A has to send another message to confirm receipt of message 2.
...and so on.
None of these confirmations are simply "nice-to-have" - if you need to be 100% sure that you're both going to attack, you need to ensure that every confirmation you send has been received by your peer.
You have that confirmation of agreement in the third round of messages. Receipt of the fourth message proves that both sides have knowledge and have agreed, aware of the other's intent. That's the message you have to prove has arrived for surety. Once you've done enough messages to prove message #4, you're good to go.