What is the answer to stopping it ?
There is no answer to completely stopping it. But there are things we can do to help prevent most from happening.
1. The government is already tapping our conversations and watching/arresting those who show up on the radar and commit certain types of crimes.
2. We have to make every square inch of this country an uninviting place for terrorists. Things like taking control of our borders. Keeping track of those who come here on visas. Arrest and/or kick out those with expired visas. We need to get our sovereignty back.
3. Arrest those who are here illegally and deport them. Create a law if they are caught a second time here illegally we cut off their nuts/put them to death/something and anything to make it so they never want to get caught.
4. We have to enforce the laws we already have on the books to the fullest extent. Or create new laws that overtly punish those who come here illegally. And create and/or uphold the laws we have to the fullest extent that is you are caught doing/or with certain terroristic things we will uphold the laws to the fullest extent.
5. Bring back the death penalty. You kill us we kill you. Period.
6. Get rid of all the Liberal judges in our court system and bring in judges who uphold our laws and don't have a bleeding heart.
7. Get rid of gun free zones.
8. In possession of an illegal firearm? Or illegally possessing a firearm? Or both? Punishment to the fullest extent of the law. Automatic. No court necessary like the "three time offender" rule or the "Use a gun, go to jail" rule.
9. Arm the people. Let us law abiding citizens protect ourselves.
10. Force Hollywood to start make propaganda films showing radical muslims cutting off innocents heads, raping women, keeping women as slaves, etc. Something, anything to at least show the citizenry what these radicals are capable of. But not just once every year or two or ten. These need to run constantly on the TV and radio and YouTube and Facebook to remind us of 9/11, Paris, Benghazi, 1972 Olympics, etc. People need to stay vigilant.
This is just my short list. I am sure everyone here can add an item or two. We cannot stop this from happening completely. But if we make it distasteful for them to try anything here then we accomplish our realistic goals and prevent as much as we can from happening. The biggest thing is we cannot rely on just government to take care of us. We need to take an active part to protect ourselves and our families. The people need to be a part of the solution. We all need to do our part. Kennedy's speech "Ask not what your country can for you. Ask what can you do for your country" comes to mind here.
To me, the biggest reason it happens mostly in Europe and the middle east is because all of these countries are not sovereign. Their borders are porous. Hence the one Paris terrorist that was caught came into the Eurozone through Greece. Can you imagine if Israel didn't have as much control over their borders as they do, how often they would have this happen to them?
Sorry for the long rant.