There were over 200 bombings in the US in 1970 alone, by all sorts of political groups (the Puerto Rican Liberation Front, the Jewish Defense League, the Weathermen, the Black Panthers, anti-Castro groups, white supremacists, etc., etc.) There was essentially no religious terrorism; that came in the 80s and 90s. But let's zoom in on 1978 onward, after the crazy period we inaccurately call "the sixties". First, a count of Islamic terrorist acts worldwide:

This is incomplete, because the database contains over 400 Islamic terrorist groups, but only let me select 300 groups at a time. (Al Qaeda is one of the groups not included here.) Also, this doesn't list any acts committed without direct supervision from a recognized terrorist group, nor acts whose perpetrators were not identified (about 77% of the database, estimated from a sample of 100, with the vast majority of those unknowns in Muslim countries). But we can see there's an increase after 2000.

Now let's look at terrorist acts of all kinds in the US:

We see a dramatic drop in terrorist acts in the US after 2000. Sampling them, I found that except for less than a handful of white supremacists, there are only 3 types of terrorists still active in the US: Nutcases, animal liberation activists, and Muslims. If we exclude cases of property damage (which has never terrified me), it's basically just nutcases and Muslims.

Going by body count, it may still be an increase, because even if you exclude 9/11, just a handful of Muslim attacks still accounted for 50% of US fatalities in terrorist attacks from 2000 through 2013. But counting incidents, by 2005 there were about 1/3 as many per year as just before 2000. From 2000 to 2013 there were only 6 violent terrorist attacks in the US by non-Islamic terrorist groups that were not directed solely at property damage, resulting in 2 fatalities over those 14 years. Violent non-Islamic organized terrorism in the US has been effectively eliminated.

Some of this reduction is because we've massively expanded our counter-terrorism agencies. But if that were the explanation, given that homeland security doesn't stop all of the Islamic attacks they're focused on, surely we would see more than 6 attacks by other groups in 14 years.

Much of the reduction might be for non-obvious reasons, like whatever happened around 1980. But I think the most-obvious hypothesis is that Islamic terrorists gave terrorism a bad name. In the sixties, terrorism was almost cool. You could conceivably get laid by blowing up an Army recruiting center. Now, though, there's such a stigma associated with terrorism that even the Ku Klux Klan doesn't want to be associated with it. Islamists made terrorism un-American. In doing so, they reduced the total incidence of terrorism in America. Talk about unintended consequences.

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On a completely different note, I couldn't help but notice one other glaring thing in the US data: terrorist acts attributed to "Individual" (a lone terrorist not part of an organization). I checked 200 cases from other countries and did not find one case tagged "Individual". But half of all attributed cases in the US from 2000-2013 are tagged "Individual". The lone gunman thing, where someone flips out and shoots up a Navy base, or bombs a government building because of a conspiracy theory, is distinctively American.

Perhaps Americans really are more enterprising than people of other nations. Perhaps other countries can't do the detective work to attribute acts to individuals. Perhaps their rate of non-lone wolf terrorism is so high that the lone wolf terrorists disappear in the data. Perhaps we're more accepting of "defending our freedom" as an excuse for shooting people. Perhaps psychotic delusions of being oppressed don't thrive well in countries that have plenty of highly-visible oppression. But perhaps Americans really do have a staggeringly-higher rate of mental illness than everyone else in the world. (Yes, suspicious study is suspicious, but... it is possible.)