Crossover Shipgirls: Building a Dimensional Empire from Scratch

Setsuna's ambitions went far beyond conquering a single island nation.

Japan was just a stepping stone—a resource-poor land reliant on imports for coal, metal, and rubber, with little in the way of advanced technology during the Taisho era. Its primary industries were fishing, agriculture, and textiles.

At this time, Earth was undergoing intense transformations.

• The British Empire, having completed the Second Industrial Revolution, was at the height of its global expansion.

• The United States was transitioning from an agricultural society to an industrial power in the post-Civil War golden age.

• The Qing Dynasty was struggling to survive amid internal and external crises.

• In Africa, a land rich in cultural diversity and resources, faced both the challenges of colonial exploitation and internal conflicts.

If left untouched, this world would likely follow the same trajectory as Earth in Setsuna's previous life: two world wars, a Cold War, and eventually a unipolar superpower with competing secondary powers.

In the grand scheme, the existence of "Demons" in the Demon Slayer Universe was nothing more than a minor footnote in the development of civilization.

But Setsuna had no intention of letting this world progress naturally. His plan was to take full control and transform it into a resource hub for his own civilization's growth.

This wasn't about a short-sighted pillaging spree. Instead, it would be a sustainable exploitation—leveraging advanced technology to boost productivity while systematically extracting resources.

Two days later.

News from Sasebo Port shattered the shogunate's last vestiges of hope.

"According to reports from the Demon Slayer Corps and local witnesses, the Alliance Fleet recently annihilated all demons near Kyushu."

"They leveled Sasebo Port, and fires burned for days before they were extinguished."

"Residents claimed to have seen explosions as bright as the sun at night and enormous mushroom clouds, with the surrounding terrain permanently altered…"

The accompanying photographs vividly displayed the Alliance Fleet's overwhelming power.

The shogunate had never cared much about demons, seeing the Demon Slayer Corps as nothing more than a civilian militia.

But the ability to flatten mountains and erase entire cities from the map? That was very real.

For the first time, they fully grasped the chasm between their military capabilities and the might of a modern civilization.

Kyoto—or even several coastal cities—could vanish in an instant if the Alliance chose to attack.

In the shogunate's halls, the generals, advisors, and ministers sat in silence, staring at the photos with cold sweat dripping down their faces.

There was nothing to say. They couldn't even imagine resisting such power.

Thud.

Thud.

Setsuna walked into the hall once more.

"Well? Have you made your decision?"

He looked at the officials expectantly.

"..."

"We accept your terms, General," the shogun said after a long pause.

His expression was grim as he signed the document and stamped it with his seal.

"The shogunate agrees to become a tributary state of the Starsea Alliance. From now on, all national affairs will follow the orders of the suzerain."

"Good. It seems you've made a wise choice."

Setsuna took the signed document.

"I will establish an administrative office here. Every quarter, we will collect resources according to the stipulated proportions."

"Prepare them in advance. Delays will not be tolerated, and the consequences will be yours to bear."

His words were resolute, and the officials could only nod repeatedly in agreement.

Only after watching the fleet depart Tokyo Bay did the court breathe a collective sigh of relief.

"By the way…"

Staring at the departing ships, one official suddenly remembered something.

"Their heading… is it toward America?!"

After incorporating Japan into the tributary system, the fleet resumed its journey.

Sailing eastward across the Pacific, they reached the western coast of the United States.

At this time, there was no United Nations to mediate, so Setsuna had to conquer one country at a time.

"America is a land of freedom and equality! We have just escaped the tyranny of the British Empire and will never bow to any power!"

The United States sent a grand armada of ironclads and armored cruisers to intercept the Alliance Fleet.

One day later, the American navy was reduced to molecules, with all ships stricken from the naval register.

The fleet advanced to Washington, D.C., and added another tributary state to the list.

"The Royal Navy is invincible! No one challenges the might of the British Empire!"

At the mouth of the Thames, the world's strongest navy—His Majesty's Royal Navy—challenged Setsuna.

Two days later, the Union Jack was replaced with the Alliance's eagle banner flying over Buckingham Palace. Tributary state +1.

"Vive la France! France will never surrender!"

Half a day later, Richelieu's battleship sailed down the Seine and shelled the Palace of Fontainebleau. The French nation waved its signature white flag. Tributary state +1.

In just one month, most of the Earth's major nations had either succumbed to diplomatic pressure or been "persuaded" through physical negotiation to become tributaries of the Alliance.

The fleet had unwittingly achieved a monumental feat: World Domination.

"Commander, how do you plan to manage this world in the future?"

Richelieu looked at the map, where the newly subjugated nations were marked. Her headache was palpable.

Each country was at a different developmental stage: some industrial, others agrarian, with diverse languages and cultures.

Even in modern times, humanity had never established a unified world government. Doing so in the early 20th century seemed even more improbable.

"Of course, we'll let them stick to their respective roles and become our massive resource base," Setsuna replied.

"When we reach the space age, we can send a small team to unify this world and establish a global Earth government."

He patted Richelieu's shoulder.

"Mission accomplished. Prepare for our return."

"Now we have the resources of two worlds. When we enter the interstellar age, who knows? We might end up developing two—or even more—universes!"

Whoosh.

With the opening of the dimensional portal, Setsuna and his team returned to their original world.

The blue sky and clear sea greeted them with a gentle breeze. The air quality on this pristine Earth was leagues ahead of the industrialized world of Demon Slayer.

That place was still stuck in the early industrial era with no concept of environmental protection. The PM2.5 levels were probably off the charts.

"We've been gone for over a month. I wonder how everyone has been doing," Setsuna mused. "Did Yat Sen manage the base well? She should be fine; it's not like anyone could attack us…"

"Hey~ Commander, you're back!"

Suddenly, a lively greeting echoed from above.

Looking up, they saw a silver-haired, cat-like ears shipgirl hovering midair, rocket-pack-like thrusters strapped to her back, spewing bursts of plasma to keep her afloat about 30 meters above the sea.

"Yukikaze?!"

"How are you flying?!"

The five shipgirls who had accompanied Setsuna looked up in utter disbelief.

"Didn't Commander give Akashi and the port all that technical data from the Gundam world? When we were building the Gundam, we thought we'd tinker with its tech for fun!"

"Some of it wasn't very practical, but the atmospheric thrusters and energy weapons turned out to be pretty interesting!" Yukikaze explained, adjusting her thruster pack to descend and land gracefully in front of them.

"…"

Setsuna recalled how he had provided the port with advanced blueprints. The shipgirls of Azur Lane had seemingly gone on a creative rampage, relentlessly pestering Akashi to experiment with new gear.

They had attached Gundam-style thrusters to themselves, aiming to replicate aerial mobility. Early prototypes were less than reliable—barely capable of maintaining ten meters of flight before crash-landing into the sea, leaving many shipgirls dizzy and battered.

But the latest iteration was a massive upgrade: smaller, more powerful, and capable of propelling even thousands of tons of destroyer-class ships into the air.

"So, all of you can fly now?"

Richelieu asked incredulously.

"Not all of us. Only destroyers can use them. Big sisters like you? Too heavy!"

Yukikaze proudly placed her hands on her hips. With a gleeful smile, she reactivated the jetpack, shooting into the air and leaving a fiery trail as she drew a cat face in the sky.

For the first time, her light weight felt like a badge of honor. Usually, destroyers envied cruisers and battleships for their firepower. Today, they had the upper hand.

Setsuna gazed at the sea in the distance, where more destroyer-class shipgirls zipped through the air.

With their jetpacks and manifested riggings, they could now provide aerial support, conduct naval bombardments, and execute amphibious maneuvers, essentially becoming tri-environment combatants.

Meanwhile, the battleships and carriers below could only watch with envy.

"…"

It was hard to deny the benefits of a robust military-industrial complex. The shipgirls had taken Gundam's tech and seamlessly integrated it into their world.

Although this wasn't the direction Setsuna had imagined for their development, he had to admit it was impressive.

"Rocket Troopers Pro Max edition. Shipgirl Flight Corps, huh?"

"This feels suspiciously like Azur Lane infiltrated Gundam's labs and raided their experimental designs…"

After the amusing interlude, the shipgirls soon noticed the return of the expedition team. They rushed to greet them warmly and celebrate their return.

The shipgirls who hadn't gone on the mission crowded around, eager to hear tales of the otherworld.

From Richelieu annihilating Doma with a single shot to the aerial bombardment of Muzan's demon army, the re-enacted Black Ship Incident, and the eventual conquest of the entire world, every story left the listeners wide-eyed and thrilled.

"Wow! You didn't bring me?! Commander, I want to go next time too!"

"I could've taken on that so-called Royal Navy fleet all by myself!"

A few eager destroyers grumbled and protested noisily.

Setsuna reassured them, promising to bring them on the next expedition.

"Now, let's review our operation."

Gathering the vanguard team, Setsuna began the debrief.

Every operation, whether a naval sortie or a cross-world expedition, required a comprehensive summary to account for resources consumed, gains acquired, and objectives achieved.

"The technological level there isn't high, and their supernatural or magical abilities weren't particularly impressive."

"These are the strongest things we brought back: a Breathing Technique manual and a Blood Demon Art manual. Oh, and this—"

Setsuna placed the two manuals and a sealed vial of Muzan's blood on the table.

"—a sample of Muzan's blood for Akashi to study."

Curious, Yukikaze flipped through the Breathing Technique manual.

"Breathing Techniques? Swordsmanship? Can I learn it?"

"…"

"You can, but do you really need to?"

Setsuna chuckled.

"These techniques are meant to enhance human physicality. But you, a destroyer, no matter how much you train, aren't turning into a battleship, are you?"

"You're engineered constructs—your combat potential depends more on engineering and technological advancements than personal skill."

He gestured to her jetpack.

"It's better to upgrade your gear than to practice fancy sword moves."

"Tch! Who needs battleship gear? They can't even fly!" Yukikaze pouted, but the reasoning was sound.

"More importantly, our greatest gain was securing total domination over that world."

"The Demon Slayer Universe is now our resource production hub, supplying both the port and this world with materials."

Setsuna laid several diplomatic agreements on the table, signed by the nations of the other world.

"Judging from how you handled things… it sounds pretty violent?"

Ning Hai tilted her head quizzically.

"I wanted to collaborate peacefully, but the problem was their lack of a unified world government. Even if I wanted a proxy to manage things, it's not like I could count on the Qing Dynasty."

Setsuna grinned.

"And working with those capitalists? Forget it. They're scheming backstabbers."

"So, I cut the Gordian knot. Quick, decisive action to flatten them and establish a new order."

He shrugged as if it were the most natural thing in the world.

"So, we just wait for them to pay tribute? Every few months, we collect resources?"

Ping Hai asked, her tone uncertain.

"Exactly. But first, there are still some other things we need to handle."

"We're aiming for sustainable pillaging, after all. Sustainability first, then pillaging."