That is one reason why the Pentagon now ranks cruise missiles the number one proliferation threat. Meanwhile, no one knows how to shoot the things down.
Cruise missiles fly too low for most radar systems and they can manoeuvre at high speeds.
Knocking out or evading a smart cruise missile is a lot like trying to dodge a bullet - except these bullets can track you when you move. No proposed Star Wars shield ever had the means to protect against such 'air-breathing' threats. Then it will be cheaper to attack than defend. This will no doubt help to start some of the 'smart wars' of the next few decades. Imagine Israel trying to fend off swarms of Arab smart cruise missiles tipped with DNA gas.