It's not just what I would do, it's what I would have done.
I would have declared war on 9/12/2001.
I would have renamed the Department of Defense back to the Department of War.
I would have identified
I would gain control of all the U.S. borders and ports of entry and established a wartime level of security on those potential weaknesses.
I would arm personnel on all U.S. military bases. It could be selective. It could be volunteers that undergo additional training. But I would not ever again want to read about a dying Ensign crawling out of a active shooter scene to raise the alarm.
I would look clearly at what the threats are, what the beliefs of those threats are, and then act to counter those threats. This could be everything from more carefully considering what individuals and groups I allow into the U.S. to Curtis LeMay style threat removal on a large scale.
I would not have my investigative services tying themselves in knots trying to not use the T word when talking about attacks. We would be honest about the source and nature of the threats.
I would end the occupation and nation building in Iraq and Afghanistan. We didn't win this war yet. The nation building is premature. We didn't negotiate with the Nazis in Germany or Imperial Japan. We did help them rebuild in the years after they unconditionally surrendered.
I would do this with enough force that for the next thousand years when some teenager popped off with a suggestion that he was interested in SJS, his family and friends would beat him until he changed his mind.
I would establish these policies in perpetuity. If they turned out to be inadequate, I would reassess.
Eternal vigilance is the price of liberty.