I think that I am probably over thinking this which has lead to me become confused as to what それ is referring to below.
To provide some additional context, the main character had just asked his friends 「みんな、大人と戦う覚悟はできてるか？」and the below extract begins after their answers.
I was originally thinking that それ was referring to the utterance 「みんなで、この世界を守り抜こう。たとえ、誰がここに攻めてきても、追い返してやるんだ！」, i.e. this was all he had to say in the first place (instead of asking the proceeding question to confirm their resolve). However, then I thought that if he was actually saying this then his utterance would have been worded differently which now leaves me a bit confused as to whether he is saying what I originally thought, or if he is referring to [みんなが笑顔で頷いてくれる].
I would appreciate if the answerer could clarify which is correct, (and potentially how the sentence would have been worded differently if they intended the other meaning).