Marx was wrong it's not that religion is the opiate of the masses, it that fear is the catnip for the masses.
Fear has several effects. First, it makes people passive. Second, it makes them easily led by those who promise to remove the source of fear. Third, it diverts them from consideration of more pressing problems.
Fear and hatred have been a feature of European history for hundreds of years and have led to repeated wars, pogroms, genocides and millions of deaths. Even in the "enlightened" 21st Century areas of Europe are still battling with their neighbors.
The companion of fear is hate. One hates that which one fears, so promoting hatred fosters fear. This has been a mechanism in use for millennia. Tribe A fights tribe B because they fear that they will be attacked first.
The US presents a more interesting case. The US has never been under a real threat, once the Revolutionary War and the War of 1812 ended. It has never been invaded. It has never fought an ethnic battle with its neighbors, although it has fought wars of conquest with Mexico and native Americans.
Nevertheless since around the 1890's it has been subjected to one sort of fear or another. Originally it was fear (or hatred) of the large number of immigrants that entered the country up to the beginning of WWI. Once fear halted immigration, the focus shifted elsewhere. The most popular theme was that "foreign instigators" were undermining the country and those that agreed with their philosophy were to be feared. The first to feel this were the labor organizers and their leftist supporters like the Wobblies. Workers couldn't be agitating for better conditions on their own, but must have been provoked by socialists or anarchists from Europe. This led to jailing of thousands of union workers and the expulsion of movement leaders like Emma Goldman. Since her family had come from Lithuania she must be a Russian agent. The Palmer raids were designed to suppress the labor movement. Even publications from the movement were suppressed as the post office refused to deliver them.
A detailed history would take too long, so I'll just mention a few other domestic examples. The one with the most impact was the internment of Japanese-Americans during WWII. After the war there were the "Red scares", the cold war, juvenile delinquency caused by comic books, gangs and "crime" and the evergreen "war on drugs". More recently the fear mongering has been more targeted. The religious are told to fear "secularism". Those in areas of high immigration are told to fear newcomers.
On the international front, we had the real threats during WWI and WWII. When these subsided the US was ready to invent new foreign enemies. This also wasn't anything new. Wars of expansion had been going on since the Spanish-American War of 1898. What sort of threat Cuba or the Philippines posed was never defined. The same tactic was used to promote the Korean and Vietnam wars. The preferred narrative was a combination of the Red menace and the domino theory. With the collapse of the USSR the US was at a loss as to who to fear.
The events of 9/11 and several other incidents elsewhere have filled the bill. These actions have provided an even better target for fear. No longer is the enemy a well-defined state like the USSR, but now it is some invisible force called "terrorism". Anyone might be a terrorist. Fear has now become universal. People have internalized it so that the most outrageous statements pass without notice. A spokesman for the TSA (Transportation Security Administration) said that 80% of people passed through airport checkpoints without problem. This meant that 20% of the population was immediately under suspicion! This level of mutual distrust has never been seen before in the US. One would have to look at East Germany or the USSR during their darkest days to see such a high level of distrust.
If we examine the risk objectively in all these cases we find that it has been widely overstated. 9/11 was the worst attack on the US ever, but in terms of lives lost and property damage it was not significant. Over ten times as many people are killed each year just in auto accidents. The destruction of the Gulf Coast by Katrina dwarfs the damage done on 9/11. Has the response been proportional to risk or damage? Do we see draconian laws regulating transportation? No. Has a proportionate amount of money been allocated to rebuilding the Gulf Coast? No. Has proportionate defensive action been taken to defend the Gulf Coast against future storms? No.
It has been estimated that the cost of dealing with 9/11 now exceeds one trillion dollars. This includes all the new security systems put in place, the rise in policing, both private and public, additional surveillance and, of course, two retaliatory wars. One situation represents a real, continuing risk, but that cannot be assigned to a specific "enemy" whille the other represents a low risk, low probability occurance, which is the perfect situation to be used to promote fear. It's also highly profitable. That trillion dollars went someplace.
So, while people have been looking for anarchists, Reds, terrorists, drug kingpins, etc. under their beds they have not been looking at the hollowing out of the infrastructure of the country, nor of the empoverisment of the middle and lower classes. While our phones are being tapped and our emails read, we aren't noticing the restrictions placed on access to information via media consolidation and intimidation of newspapers and broadcasters.
Societies which don't trust themselves, fail.