It would be easier to get a Terrorist in on Air Force One than through the Mexican border.
Mexican banditos have been running the border with the USA for a long time. There are many stories about Mexicans paying other Mexicans to get into the USA. Those bandidos watch the border and shake down anyone who wants to cross it.
There is another group that watches the border from the Mexican side. Those are the drug smugglers.
Those two groups, the people smugglers and the drug smugglers, know that if a terrorist gets trough and does some damage these border will be closed tighter than a drum. The easy money days will be over. The thing they must prevent is Terrorists getting through the Mexican Border.
A terrorist with out money or arms is pretty helpless in the USA. Those who would try to cross the Mexican border would need to have money or Arms on them.
Should a terrorist try to cross the border they banditos would take their money, kill them. If the Terrorists defend themselves with weapons they would be destroyed. In fact Killing Terrorists for Money has very likely happened many times.
It is interesting that in World War I, World War II, and the Cold War spies and saboteurs never tried to enter the USA through the Mexican border. It was as open then as now.
Germany tries to sabatuers from subs off our South Eastern Coast. We caught them. Russia brought them in as Russian embassy officials.
None ever successfully used the Mexican border even though from our side it was an open, poorly guarded border.
It has been over 2.5 years since 911 and they still have not gotten a terrorist through the Mexican border. I doubt that they even try.
You are correct, but it will only take one. In a society of thieves, there are no standards, just a larger bribe.
And to rely on criminals to protect us?