@Allistar1801:
@lordabdul writes a good post for getting yourself into the framework for creating an adventure, and I want to combine his good advice with what I said to bring that first step into a more concrete place.
War story: they're idealist graduates of the Battle Magic University (who forms future military commanders), who, after their first field assignment, witness the actual horrors of war wizardry, and the cost of fighting for their nation, as they follow orders and push into enemy territory.
Ok, fine, that is a good goal. But it won't get you very far in turning that goal into an adventure beyond giving you a framework for deciding what sort of antagonist you want to create. The next step here to concretely write down who the antagonists are, what they want, and how they plan to get it. Remember, each adventure is about how the Heroes respond to the challenge presented by the antagonist. So in this case, if you are doing a "War Story" you need to decide who the Battle Magic University is going to war with, why that foe is threatening, what that foe wants, and what that foe is doing to go about this.
As a the most simple and stereotypical framework, the BMU might be going to war with the evil Kingdom of Darkgloom, the ancient enemy of the Battle Magic University, which is determined to subdue the peaceful people of Haven. And the enemy is attacking with a large army across a broad front, in an effort to directly overwhelm Haven's defenses, pushing specifically toward the nearest cities to the border within intent to besiege and conquer them. Your new recruits are charged with defending a small village, or a road, or a bridge, as part of the cohesive defenses of Haven against this attack. The enemy, the evil Captain Snarlyface, has certain resources he will throw against the PC's in an attempt to overcome the defenses they set up. So there might be an initial probing attack by Captain Snarlyface's scouts, followed by an attack by some of his cavalry, and so forth. To succeed, the PC's need to organize local resistance and develop a systematic defense in depth of whatever they have been charged to protect. Congratulations, you just wrote an adventure. If that adventure is successful, then you can invent a new scenario based on what happened in the first one. If it wasn't something everyone enjoyed, then you can pivot to a different idea and assume that whatever challenges were presented were resolved off screen by NPCs, and move on to an aesthetic of play that your players enjoy more.
And of course, if your players are jaded and sophisticated and such stereotypes are old hat to them, then you can always throw in twists like, it turns out they are working for the bad guys and they have to decide where their loyalties lie, or the orders they've been given are absurd, and they have to decide to violate the letter of their orders in order to achieve the thing they actually want, or whatever.
Spy story: they're patriotic graduates of the Scrying School (who teaches you the art of intelligence gathering and magical tradecraft)... during their first simple mission tracking emissaries/traders visiting the capital, they step into a conspiracy that threatens the very source of magic that powers their nation.
Which again, is a perfectly fine starting point, but this idea will go no where unless you decide who the antagonist is, what that antagonist wants, and and what the antagonist has done and will do in order to achieve this objective. In other words, you need to know what that conspiracy actually is.
So here, this being a spy story, the more twists you put into it probably the better. The twists are actually the thing you are going for here, because a good spy story usually works around shifting perspectives about what is going on. At the very least, it ought to take some time to reveal that the actual purpose is to attack the very source of magic that powers their nation, but there also might be surprise allies. For example, one of the emissaries is actually a double agent working for an allied power, but that agent is hesitant to reveal themselves to the PCs, because they know that the PC's boss is a double agent working for the bad guys and the reason they have been assigned to this case is precisely because they are inexperienced and presumably inept agents who will be unable to unravel the mystery in time. Perhaps that double agent assumes the PC's are actually subverted double agents as well. Bang, now you have the basics of an adventure.
When I'm setting up a campaign, one of the first things I do is list all the factions I can think of that are in the immediate vicinity of the starting point, and then come up with the plot each of them is undertaking at the moment to achieve their goal. This can include the BBEG whom I'm expecting to drive most of the action of the campaign, but it also will give me a wealth of side plots and quests that the PC's can partake in while the main plot is unfolding, as well as give me some idea as to what interactions with NPCs by the PCs is going to be like.
