I have to warn your outline is super risky.
Step 1: risks losing your player's interest because it starts off as every campaign ever. Without any particular star to reach for the PCs are likely to create some for themselves... or play a lot of Angry Birds whenever you don't have to spotlight directly on them.

Step 2: it's rare that a group of PCs function well inside a standard military structure, what with their tendency to want to be special snowflakes. This problem is magnified several fold if they've had free reign up to this point. They're more likely to abandon the organization and strike out after their previously established personal goals.

Step 3: there's no way to guess how a given table will respond a given stimuli. It might not even occur to them to take their vengeance against the kingdom proper, much less take that revenge in the form of conquest. What if one of them is a druid who'd prefer to see any bastion of civilization leveled to the ground? The reason "save the world" is so easy is that any group of special snowflakes will be on-board with it, because "the world" is generally where they keep all their stuff.

Step 4: I really think you should just start here. It's a good hook to get the players interested and it encourages them to make characters appropriate to the campaign. But then again, I'm just biased against railroads. I prefer to give my players a mutual goal and provide a handful of ways to achieve said goal. Cat herding is a hell of a lot easier when you just dangle a bag of catnip in the direction you want them to take, rather then building a giant reinforced thin corridor and pushing them forward through it with a broom.

Before they create their characters, have them all start with the same bit of back story. "You were all part of a mercenary band that was betrayed by the Kingdom. You are the only survivors and you've come together swearing to conquer the kingdom and rule it yourselves."