"Have you heard of the Battle of Wizna? It is a WW2 battle where around 720 Polish troops held off over 42,000 Germans. It is also called the Polish Thermopylae. The polish soldiers was in fortified bunkers and severely outnumbered and outgunned. But that did not stop them from fighting, they even managed to shoot down a bomber plane."
Will our security officers be prepared to shoot down a Mexico City-New York passenger jet?
"And regarding tunnels, you can dig tunnels but it is likely your army will sustain casualties not to mention give the defenders time for support to arrive. Like at the second Battle of Vienna. The Turks tried digging under the walls to place mines but in the end they failed to take Vienna. If there was no wall and the forces garrisoned at Vienna had to meet the Turks in open field they probably would have lost. "
Casualties, no matter how many, didn't stop the Transcontinental Railroad from being built.