What do you mean by “ground-breaking”?

^^ Introduced a new way of composing shots, filming shots, telling new narratives and never before seen themes, subject matter…

There never was an “era” of ground breaking movie makers.
There have only ever been single points in time where there was a singular film maker that did something “ground breaking”, followed by a slew of copy-cats.

There will be more “groundbreaking” movies. There will ALWAYS be more groundbreaking movies.

What would be an example of a ground breaking movie in your view? (Not being difficult. Just establishing parameters before I answer)

The Godfather, Lawrence of Arabia, Star Wars .

Ground breaking…hmmm.
Which fucker directed Earthquake?

Although I am not a fan of his, quite a few people would consider James Cameron as ground breaking in his integration of animation into his movies, and with his work in 3D.

And Richard Linkalter is in his prime. Boyhood is unlike any other movie ever made.

It is the most true-to-life movie ever made.
At the end of it, you just want to die.

Daniel Myrick and Edwardo Sanchez broke new ground with The Blair Witch Project, and I can’t think of a film since that broke new ground like that.

What about “Borat”?

Can’t think of any either since Blair’s Witch. What about Irreversible?

Or “Grimsby”. That elephant scene was pretty ground breaking.

Eraserhead…

Okay, I’m at a computer. Ragingloli is correct. There was no “era” of ground breaking movies. But here’s a small list of movies that I consider to be ground breaking—meaning things that had never been done before.

2001
Alien
Blade Runner
Witness
Gallipoli
Salaam Bombay!
Apocalypse Now
Groundhog Day
Ran
The Good, The Bad and the Ugly
Spinal Tap
Eraserhead
Elephant Man

@Caravanfan All good movies, except Eraserhead, but I agree it was ground breaking.
I might include Memento and Rashomon on your list.

Quite a few non-mainstream-USA films have been made for many years that do new and experimental things. The focus of the films listed so far seem mainly to be the few mainstream films that have also done new things, or just been very well made.

In the terms of the original question, it seems to me there are many wide fields of broken ground spreading out in various directions from the mainstream US films.

@filmfann It wasn’t an exhaustive list, just ones that I had off the top of my head that could be considered “Groundbreaking”. I didn’t like Eraserhead either but it was the weirdest movie I’ve ever seen.