Rivalry often plays a big part. Just like in the original 'this means war' idea, there's usually two sides at odds with each other. In the HP fanfic context, it might be two groups of students or different magical factions. For example, the Gryffindors and Slytherins might be in a 'this means war' type of feud over control of a secret part of Hogwarts. And there's also the theme of strategy. Characters need to come up with plans to outwit their opponents, much like in a real war situation but with a magical twist.
Competition is a common theme. It could be between characters for a powerful magical object or for recognition within the wizarding world.
One common theme is loyalty. When it's 'this means war' in a HP fanfic, characters have to decide which side they're on and stay loyal to it. There can be a lot of internal conflict as well. Another theme could be the use of magic in unconventional ways. Since it's a 'this means war' situation, the characters might have to push the boundaries of what they know about magic to gain an edge over their rivals. For example, a character might combine spells in a new way to create a powerful attack or defense mechanism.
Chapter 503, ", Zhuge-class Star Mothership", is the clearest early answer because it turns Alliance's situation into a concrete plot event. A useful checkpoint is the way the scene starts with In the midst of space warfare, the most critical capability of an... and follows through with The longer a world is under Alliance control, the rule of the....
Chapter 419, "Turning the Tide of Battle", is the clearest early answer because the buildup finally cashes out in a real clash. The setup is Before the retreat, the human army wouldn't initially destroy the..., but the pressure keeps building through However, over time, they realized that the Green Skin Orcs had....
Chapter 450, ", The Death of the Ashen Dead", is the clearest early answer because the violence around Thunder Spears becomes final instead of hypothetical. The chapter gives readers a real answer by opening on The Chief Wizard of the Ashen Dead Squad was indeed quite formidable. and ending on However, their combat power was too different; involving them in the....
Chapter 426, ", Factory Exchange of 500,000", is the clearest early answer because it turns Factory Exchange's situation into a concrete plot event. The setup is The battle at Steel Wing Star might be the largest campaign that Gu..., but the pressure keeps building through The ground combat alone was probably even more massive than the....
Chapter 370, ", Internal Strife", is the clearest early answer because it turns Malca's situation into a concrete plot event. The chapter turns that into something concrete by moving from After untold hardships, having passed through numerous military... to Ask yourselves, why would anyone follow us if they could live....
Chapter 381, ", Reverse His Mother's!", is the clearest early answer because it turns Isabel's situation into a concrete plot event. The chapter gives readers a real answer by opening on But no matter what they're called, or at what time, their opinions... and ending on Of course, just because he didn't take it seriously didn't mean no....
Chapter 480, ", Battleship Cruiser Technology", is the clearest early answer because the buildup finally cashes out in a real clash. A useful checkpoint is the way the scene starts with The monthly income from our homeland, the income from vassal... and follows through with In conclusion, it was a man-made disaster, unrelated to the level of....
Chapter 394, ", The Choice of Flying Wing Star", is the clearest early answer because it turns Pei Desi's situation into a concrete plot event. The chapter turns that into something concrete by moving from All these costs were borne by Flying Wing Star, which then obtained... to The equipment was purchased, and the personnel were specially....
Chapter 440, ", I don't want casualty numbers, I just want Gaomen Ridge", is the clearest early answer because it turns Green Skin's situation into a concrete plot event. The setup is When the Beastman Overlord truly gathered the vast Green Skin army,..., but the pressure keeps building through This hundred-kilometer battle line was subjected to a powerful....
Chapter 475, ", let's get past Mr. Gu first", is the clearest early answer because it turns Gu Hang's situation into a concrete plot event. A useful checkpoint is the way the scene starts with The returning Titus, with one-third of the Green Skins' main force,... and follows through with If 'Iron Fury Stone' wanted to take over the Dragonhawk Star Domain,....