Hasn't it always been like this?
No. It has't been always like this. The turning point was the disintegration of the USSR. The Soviets neutralized any threats posed by the US and vice-versa. Right now the US is the sole super power. And that is bad news for the US citizens, as well as for other people.
No not really, and the US is not the only superpower, while indeed it is a dominant superpower, other superpower have enough to destroy the US, Russia, China, France, UK for example has the power to do such a thing, but that's not the point.
I think what makes the US a terrorist den and target is it interference with other countries affairs to gain geopolitical power (and when it can't it goes for war against said country), example Vietnam (faking the incident of the boats and waging against Vietnam), Panama (CIA coup), Iran ( removing the democratically voted regime and putting the shah), arming Iraq against Iran, arming Taliban against local government and USSR, doing shit in several central american countries, south asian countries... ... the list is too long and I can't detail everything, but people should get the picture by now. And what I mentioned here is just one part of the equation, the other part is the shit that happened inside the US, do to capitalism practive, and anticompetitive ones ect