Earth is dealing with UFOs that travel both time and space

 That's why we are no threat at all to them. All they have to do is to go back and stop any humans before they were born. That's what they do. So, likely in any given situation there might be one key figure that they would eliminate who might want to nuke earth out of existence or anything else humans might do to piss them off.

So, almost all the movies are basically ridiculous compared to what is really going on. It's more like Humans are Ants and they are like people. What kind of threat are ants to people? It's a silly question unless you are talking about certain varieties of Ants in South America that eat everything including people and plants over wide areas of land. But, generally speaking ants are no threat to people unless you are someone being stung by a red ant.

As a five year old in El Cajon I had never seen Red ants before so they fascinated me. So, I stood on a Red Ant hill and soon I was screaming. I heard that my uncle Tommy did the same thing when he was 4 in Arizona too around 1922 or around there then as well. So, my experience was likely in 1953 when I was 5 years old.

So, though you can change time (or though they can change time) there are many problems sometimes when you do this so you have to consider what happens when you change the past, present and future of a planet or place in the universe. Unless you consider what is going to happen instead of what you didn't want to happen that can be a problem too.

So, even though many of the UFO Cultures that have visited earth since before it had an atmosphere can change time actually doing it is another thing.

But, it's about 1 million times as potent as nuclear weapons.

Imagine you have time travel and you eliminate on earth everyone who invented nuclear weapons in Germany and the U.S. and how that would change the timeline?

It could and would change everything that has happened since World war II. 

I would say that time travel was originally invented so that 1000s of years of space travel would be instantaneous more than anything else. So, you could get on a spaceship and travel for thousands of years and be there instantly basically from the moment you took off from wherever you were to begin with.