None of the 9/11 terrorists was "inserted" into the U.S. with anything like the sophistication used to put infiltrators, for example, into Iraq in the time leading up to the war. We are not dealing with people who have national resources readily available to them.
They flew in with fraudlulent visas, overstayed those visas, which, if you are a c op in FLA, you might recall was fumbled by the county mounties at a traffic stop. Then next bunch will have walked in from Mexico. They probably now live among tens to hundreds of thousands of illegal moslems in the U.S.
Dry up the vast swamp of illegals and the terrorists will stand out more, and MAYBE the donut-munchers will take the time to see if they are DRIVING here legally.
Make it 100X harder for them to get in, and you improve our chances of stopping the next attack by 100X or more.
Israel has proven that, under vastly more difficult circumstances, it is possible to keep terrorists out. The naysayers here are more interested in building and expanding a domestic police state because we do not defend our borders.
How do you know? How do you know they won't come in through Canada? How do you know they won't come in via Clinton's agreement to take Somali refugees? How do you know they won't be American born Muslims getting their orders via the internet?