More than five years ago, I started reading a sci-fi book about advanced humans making first contact with (definitely pre-industrial but might specifically be iron-age, medieval-era) humanoids. I don’t think it’s a pre-2000 book.
Humans either arrive in the alien star system by chance or as a fleet that uses FTL but one ship accidentally goes bonkers and warps to the wrong destination (being the alien star system).
The aliens are probably humanoid but they, for sure, are taller than humans. This was because of the planet having lower gravity.
The aliens are split into two divided cultures. The first culture lives in a very cold climate while the second culture lives in a desert climate. The humans initially side with the first culture but that may change later on in the book.
Finally, there was one scene where the humans were on motorcycle-like devices (maybe hoverbikes) and had firearms. As the humans rode past, they fired and mowed down a culture's army.