When looking at the 9/11 attackers, a few things seem pretty obvious to me in terms of ways we actually could have prevented the 9/11 attackers.
The first thing that needs to happen is they can’t have a safe haven anywhere in the world, and any countries that are safe for terrorists to host their training camps need to be heavily scrutinized. Stabilizing regions like Afghanistan is a matter of national security.
Reducing the reasons why someone would want to attack another country is a tried and true method to reduce terrorist attacks. Programs like the Peace Corps and USAID are effective anti-terrorist tools.
Saudi Arabia reissued passports to the terrorists before they came to the United States. Countries like Saudi Arabia which are known to harbor financiers of terrorism need to be heavily scrutinized and their citizens need extra scrutiny by the State Department before they are granted visas. Not just Saudi Arabia but all Gulf States.
When someone is reported to the police to be supporting terrorists, take them seriously.
Visa-free entry for democracies which we share criminal record data with is a reasonable policy that doesn’t endanger American lives. The ESTA would not have hampered any of the 9/11 highjackers. Visa-free entry for all European Union citizens and other allied democracies. ESTA does not prevent terrorism.
Countries with developing economies and very low rates of terrorism should not have large barriers to travel to the United States.
Visa refusal rate and overstay rate have little to do with countries that are the most likely to be the source of violent terrorist attacks. I am doubtful visas are an effective tool for reducing terrorist attacks.
Conflict breeds more conflict. Hatred breeds more hate. The path to world peace is through communication, trade, dialogue, and building friendships across nations.
References:
Foreign Aid as a Counterterrorism Tool: More Liberty, Less Terror?