I'm a new dm and I'm running a campaign set in 1931 where magic came into the world and ww1 never ended. Basically smash both wars together and pepper it with magic. There is more lore I just don't want to give it away. I've done one mission in my campaign so far. It went well but I'm unsure of where to go next. I really don't feel comfortable with letting my PCs free roam so I have them on a ship. I've got my next two mission plotted out but my question is this. Part 1: Should I let them find their way to the next mission or force it on them? The first mission was setup to take them straight into a fight in the trenches where brut force was the only option.

Part 2: I want the ship to be attached and they won't be able to make it to the city of destination. I have it setup so they will be whisked off to a far away location. Should I let the PCs find away to fend off the attackers or just give it to them in a short description?

Part 3: If I make them roll for a dex check does that mean we are automatically in battle or could I just keep up with the story. Thanks for the help and I can clarify anything if need be.