Railroading vs Derailing? /u/IncredulousBob DnD: Roll for Initiative!

I’ve recently joined a DND group for new players, and my goal is to eventually become a DM, but there’s one thing I’m worried about. Everyone knows it’s bad DMing to railroad your players through whatever story you’ve come up with, but is it also considered bad form for the players to go out of their way to derail the campaign? Obviously everything isn’t going to go completely the way I envision it because the players don’t know the ins and outs of the story like I do, and I’m fine with that. But there’s a difference between doing something that makes sense in-game that I wasn’t prepared for, and deliberately breaking away from the story to do something completely unrelated.

For example, if the campaign revolves around the party hunting down a dragon, it’s always a possibility that the players might decide to join up with the dragon instead of slay it, and I would roll with that even though it wasn’t the direction I expected the story to go. But if, instead of following the dragon questline, they said “Yeah, that’s cool. So anyway, I attack the king and claim his throne for myself. Now let’s go invade another kingdom!” is that going too far? At that point they’ve taken the campaign I already have planned and turned it into something I have no notes, questlines, character interactions, or anything else prepared. So am I able to tell him no, we’re sticking with the campaign I have planned, or do I just have to accept that we’re doing this now?

I know this probably isn’t going to happen often, but I still want to know what’ll be expected of me if and when it does.

I’ve recently joined a DND group for new players, and my goal is to eventually become a DM, but there’s one thing I’m worried about. Everyone knows it’s bad DMing to railroad your players through whatever story you’ve come up with, but is it also considered bad form for the players to go out of their way to derail the campaign? Obviously everything isn’t going to go completely the way I envision it because the players don’t know the ins and outs of the story like I do, and I’m fine with that. But there’s a difference between doing something that makes sense in-game that I wasn’t prepared for, and deliberately breaking away from the story to do something completely unrelated.

For example, if the campaign revolves around the party hunting down a dragon, it’s always a possibility that the players might decide to join up with the dragon instead of slay it, and I would roll with that even though it wasn’t the direction I expected the story to go. But if, instead of following the dragon questline, they said “Yeah, that’s cool. So anyway, I attack the king and claim his throne for myself. Now let’s go invade another kingdom!” is that going too far? At that point they’ve taken the campaign I already have planned and turned it into something I have no notes, questlines, character interactions, or anything else prepared. So am I able to tell him no, we’re sticking with the campaign I have planned, or do I just have to accept that we’re doing this now?

I know this probably isn’t going to happen often, but I still want to know what’ll be expected of me if and when it does.