Yes, but when you crash land on the planet, you and the crew have been sent back into the planet's past, to become the ancestors of the people who would otherwise disappear.
So when you are sent to the past, you and the rest of the crew are the only people on the planet. You'll have to make do with them. But not to worry... those Star Trek crews had some VERY nice looking women. And you're right, there wouldn't be any wars unless of course some Klingons or Romulans happened by.