There are weapons that are so devastating, they can make the land they hit uninhabitable for decades, centuries, sometimes forever. My ancestors used them.
I grew up on a space stations - a kind of large ship in space. When I was almost 18, me and almost 100 others were sent back down to the planet's surface to see if it was safe for us to live there, yet, but humanity repeated it's mistakes about 5 years later and the planet is effectively dead for good. We were able to escape in another ship.
I know that must sound really unbelievable coming from your time, but in the future, my time, there are ships than can travel through space.