Buchanan is a conservative on immigration. He opposes amnesty and unending mass legal immigration. He supports conservative comprehensive immigration reform,. He would greatly reduce immigration overall.
So how would Buchanan-favored policies have prevented 9-11? It’s simple; Buchanan policies on immigration (i.e. sane, conservative policies) would have made it much less likely that the hijackers would have ever been allowed in the country in the first place. They would have also made it less likely that garbage like the Boston Marathon bombers would have been admitted.
You have to remember one thing — Muhammad Atta was not here as an immigrant. He was here on a tourist/business visa.
Did Buchanan say that he would not allow Arab students to come here to train or to tour or to do business? Can you cite me a quote?