This is easy to say when you're from an island-nation. The few non-island nations without border control are all third-world hell holes with major terrorism problems.
What happened to the EU? Is it not also made of 'non-island nations without border control'? Sure, it has external borders — but only around a whole continent.
The majority of terrorists are second and third generation immigrants. Border control or travel bans won't stop them as they already here and would not have prevented all terroristic attacks.
Trump's travel ban or lidar-based border control wouldn't even have prevented 9/11 as most came from a country that is of economic importance.
Maybe Luckey can sell its technology to Saudia Arabia and Trump can then boast of the big money deals he made. In the mean time SA can fund European mosque's that calls not to integrate, to resist western society/ideals or democracy. That is big part of the problem.
I don't believe for one bit that this has to do with "security" or "keeping troops save". There are easier things you can do that would improve that.