The wall is completely ineffective. It doesn’t stop tunnels, it doesn’t stop boats, it doesn’t stop swimmers in the rivers, it doesn’t stop people with a ladder that is 1 ft taller than the wall.
Walls don’t work, but good policy does.
Understanding the incentives for why people would want to move past the law, understanding how those incentives drive bringing together expertise & resources to overcome our border defenses, and finally, understanding how to disrupt those incentives.
That is treating the problem.
Yes they were; in both cases, of poor intelligence gathering.
In the latter case specifically, it was a false-sense of security in the intelligence agencies, brought on by having caught Ramzi Yousef, the mastermind behind the 1993 WTC bombings, and not seeing OBL as a “big deal” because his organization had killed <50 Americans up to that point.
It was also infighting in-between the agencies who weren’t sharing information, and overlooked chances to question and deport the Hijackers. Like the 20th would-be hijacker managed to be.
It was the lack of imagination of officials who claimed they couldn’t imagine planes being used as missiles, despite a plane having twice been used to crash into the White house, a 1993 Fed Ex hijacking and that of Air France Flight 8969 having been executed with the same motive.
9/11 was a symptom, of intelligence being fragmented, government-run airports who under-prioritized security, and legacy policies that treated terrorist acts on airplanes as being either hostage crises, or bomb threats.
It may have also been the common strategy-- airline policy to intentionally turn over control of an aircraft to the hijackers, but that’s harder to know for certain.