All you need is oxygen and hydrogen. Both of those are actually plentiful on Mars.

And the machinery to gather them etcetera etcetera... Just because the raw material is there doesn't rocket fuel make. And I'd be hard pressed to say that oxygen is plentiful on Mars. There is some oxygen.

"And the machinery to gather them"

That machinery can be sent gradually and ahead-of-time.

"Just because the raw material is there doesn't rocket fuel make"

No, but it means that such fuel is at least possible.

"I'd be hard pressed to say that oxygen is plentiful on Mars"

The current understanding is that there's millions of cubic meters of water ice on or near the Martian surface (with even more suspected further beneath the surface). Sounds pretty plentiful to me.

Neither of those are plentiful on Mars.

Mars has lots of water ice, which is made (almost) exclusively of - you guessed it - hydrogen and oxygen.

So yes, both of those things are plentiful on Mars. Maybe not to Earth standards (or the standards of, say, Ceres or Enceladus), but certainly abundant compared to our own moon (at least as far as current information tells us).

It's all relative. If you already have enough energy to crack water, fueling a rocket is not a worry.