So, now I cannot be safe in large cities because they are terrorist targets?
I have been around dangerous cities all of my life. We have a saying: It ain't no fun unless you are a little scared.
The odds of being killed by a vehicle when you are crossing the street is more likely than death by firearm. The odds of dying in a airplane crash are probably larger than dying in a ''terrorist event.''
You cannot stop one man (or a few) with a bomb. There are a lot of armed state and federal police in US airports today. So far, (knock on wood,) there has been no major incident at a US airport. I think Brussels airport was a one of a kind incident, just as the WTC 9/11 incident was. Brussels had warning of danger and didn't act on it just like the US government did of a potential attack that resulted in 9/11. Easy to say in retrospect but neither warning was clear and actionable.
If there were trained soldiers deployed in places like airports and large public gatherings it wouldn't really bother me as long as they limited their actions to protecting the public. The Posse Comitatus Act limits the use of federal troops for this but the National Guard while under a state governor’s command could perform these duties lawfully. Maybe they would have spotted the terrorist bombers or maybe not ...
Busy International Airports are a special risk and hard to defend
https://en.wikipedia.org/wiki/Lod_Airport_massacre
One of the most heavily patrolled and secured airports in the world.
There are too many loose cannons (literally) running around with handguns and assault rifles today -- everywhere.