What does a war between the US and Iran mean to the stock market?

Nothing unless it results in negative impacts to the investment decision-making process that professional investors (for they are the financial markets) employ. If pro investors value their livelihood, actions taken must be rooted in some form of justification for said action. And that justification must be rooted in an acceptable methodology such as impacts to the inputs into the valuation model for stocks used. Those inputs are, primarily, earnings, interest rates and risk.

Not wildfires in California nor brushfires in Australia nor famine in central Africa nor climate change nor war in the Middle East matter UNLESS such events result in impacts to the methodology employed. This is why the financial media trained the journalistic art of cause and effect (for every event there must be a cause) mislead the average investor when they report stocks went up (or down) today because... In the professional investor dominated financial markets the "because" is not the news event unless the event becomes the catalyst for change to impacts to the methodology employed. That is why a war between the US and Iran does not matter unless...