First, it could be that you believe that war is already happening. In this case, Israel and other opponents of Iran might think that the status quo was a shadow war, and they are just continuing the war in a way that is to their advantage.

Second, even if peace technically exists at the moment, one side might believe that the other is moving toward war, and if allowed to complete preparations, will be significantly harder to overcome when they start a war in the future. In that case, preemptive war might be thought to engender peace in the medium or long term.

We don't have to agree with these casus bellis to acknowledge that they are at least superficially reasonable justifications, presuming they fit the facts.