Which group of people cause the biggest harm to society? Are Mexicans taking your job? Are Muslims causing terrorism? Maybe the Jewish are holding back your superior race. Or it could be single moms sucking up all the social services. All of these groups have been accused of being "The Problem" in societies at one point or another. It's a common tactic on the right to blame a group, often of a lower socio-economic class, for all the problems a country or community faces. The idea is that if everyone agrees that a certain group is an issue, they will give their leaders unlimited power in stopping the appointed threat. There are obvious examples like Hitler's campaign against the Jews, but we can see the same thing happening all the time, even without the goose-stepping.
As tired as the comparison may be, the right's argument on Muslims is identical to the Nazi's argument about Jews. And I'm not just talking about in America either. All around the world, there is an increase in anti-Muslim rhetoric. In an incredible feat of logic, the main argument is: ISIL = bad, and ISIL = Muslim, so therefore Musilm = bad. By letting yourself believe a terrorist group poses an existential threat to your country, you allow your fear to take control of a country. This "other" figure acts a foothold for fascism to grow as fear and unrest spread.
Ask someone if we should've let in more refugees during World War II, and they're likely to say yes. You're not going to hear the argument that we shouldn't have because that could've let in German spies. It seems almost horrifying that we didn't let people in on the off chance they could be spies, but the same thing is happening right now. Your house gets blown up and then you're told you can't enter a country because you might help the people that blew up your home. It doesn't really make sense if you apply any thought to it, but that's also the case for the argument that foreign terrorism is our biggest threat. You're more likely to be killed by a domestic Christian terrorist, but there's no argument to fight it because it doesn't fit the narrative. White Christians are the majority and so they would never let themselves become targets for fascist crusades.
Fear is a strong thing. Always be wary of the "flavor of the week" group to hate. We're being told stories all the time, and it's our responsibility to spot the narrative and make decisions based on what we know rather than what's said to us. In a world where so much is happening all the time, news sources have to choose what they're presenting. You're never going to get the full picture from just one source. History often repeats itself. Paying attention is the only thing to keep us from the past.