Moreover, such a message has absolutely been sent privately to certain parties - the sitting governments of Iran and Syria at a minimum, and such government as may care in Lebanon. It is these who are tops on the likely list of formal state support for terrorism. One message that does not need to be sent is that the last two occupants of the top of the list, Afghanistan and Iraq, no longer are. That speaks for itself.
There has been so much talk of "war" on terrorists from so many people, for so long, to so little effect, that these belligerents are to be forgiven for assuming that this time is no different, even after the fall of two host governments. It is slowly dawning on the Arab world that this is the real thing, and that angry men with guns can no longer hide behind our former respect for religion and national sovereignty. After all, the worst that can happen already has - they've been killing Americans on American soil, and the Europeans have sniffed in disdain at the impropriety of the whole thing. That last used to matter - I'm not really certain why. It doesn't now.
1. Institute martial law until perps are caught and the true nature of future threat is identified
2. Incarcerate/internment camps for many/all Muslims
3. Severe economic disruption with mass layoffs, stock market crash and huge loss of wealth
4. Possible internal civil war as the left/right disagreement erupts into condemnation and anger
5. Possible secession of state(s) in aftermath as a result of #4
6. Response in kind to perpetrators country including nukes or massive bombing
7. Massive world-wide condemnation of US response, loss of world-wide allies for decades.
No matter who the President is at that time, if the response in kind does not occur, then he will not be President for long. Our country will cease to exist as it is today, it could recover in the future with good leadership, but that would be seriously in doubt. The situation would be analogous with the Civil War when it took 50-60 years to heal the wounds and move on.
This is why the entire scenario is so important and why it is so frustrating to see the left politicize this situation so much.