The physiological structure of ninjas, combined with the pressure brought by the tense environment of the Sengoku Period, resulted in the growth paths of most ninjas from major clans being quite formulaic.
Basically, they would become genin at four or five years old and chūnin around the age of eight.
Therefore, for these clans, as long as it wasn't truly a life-or-death crisis, they would rather wait a few more years for a chūnin to emerge than send a genin directly to fill the front lines.
As for waiting until they became jōnin before sending them to the battlefield?
Don't be ridiculous. Whether someone could become a jōnin depended almost entirely on talent. Out of ten chūnin, there might not even be one who could reach that level.
Furthermore, for a chūnin, the battlefield was the best training ground. The pressure of life and death could squeeze out a ninja's full potential—something a training field could never replicate.
This also meant that most people during the Sengoku Period would be considered absolute geniuses in the peaceful era of later Konoha.
Someone like Uchiha Itachi becoming a chūnin at ten would have been completely normal in this era, while prodigies like Hatake Kakashi becoming a chūnin at six were not particularly rare.
The second—and most important—reason was something Kure only realized after coming to this world: what exactly the battles between ninja clans in this era were truly about.
For the glory of the clan?
To unify the Land of Fire?
For revenge?
Both yes and no.
Those were only secondary motives, not the fundamental cause behind the conflicts between the major clans.
The real reason—the driving force behind more than a thousand years of war—was the struggle for resources, especially population resources.
If one thought about it carefully, it became obvious that ninjas and the people attached to their clans had already transcended the common civilian class of this world. In essence, they were all non-productive members of society.
And their numbers were far from small.
Just among the well-known ninja clans in the Land of Fire that Kure had heard of, there were dozens. Among them, more than ten could be considered great clans.
He didn't know much about the others, but for the Uchiha clan alone—according to the records he had secretly glanced at on the Great Elder's desk—
There were already six or seven thousand ninjas living near the clan territory.
Meanwhile, thousands more were stationed in the west, the east, and along the front lines.
If family members, children, and ordinary clansmen were included, the non-productive population of the Uchiha clan alone reached twenty to thirty thousand.
None of these people were directly engaged in production.
In a world where civilian productivity was still very limited, feeding the Uchiha clan alone would require more than a hundred thousand civilians.
But a ninja's life was not as simple as just having enough to eat.
Their standards of living far exceeded those of ordinary civilians, not to mention the enormous resources consumed by years of continuous warfare.
The number of civilians under Uchiha control was generally maintained between four and five hundred thousand.
Roughly calculated, if the ninjas, clansmen, and affiliated personnel were added together, both the Uchiha and Senju clans each had nearly tens of thousands of people.
Other great clans such as the Hyūga, Kaguya, Sarutobi, Shimura, Nara, Yamanaka, and Inuzuka also had populations exceeding ten thousand.
Medium-sized but well-known families were smaller than the great clans, but they still had over a thousand members.
Altogether, the non-productive population in the Land of Fire probably exceeded one hundred thousand.
If calculated using a ratio of one to thirty, the demand for civilians would reach into the millions.
After arriving in this world, Kure realized that reality was completely different from the theories many fans had proposed in his previous life.
The Naruto world was roughly similar in size to Earth—or perhaps even larger.
Therefore, despite centuries of warfare, the Land of Fire still had more than ten million civilians.
However, aside from ninjas, there were also many military generals and wealthy merchants among the civilian population.
Those ten million people simply weren't enough to satisfy the needs of all the major ninja clans.
After all, the expenses of maintaining a ninja were enormous. They couldn't possibly be supported by just thirty civilians.
Of course, this estimate was somewhat exaggerated.
Ninjutsu could significantly increase productivity, and during periods of ceasefire many ninjas would assist in agricultural or industrial production.
This also explained why the Uchiha, Hyūga, and Kaguya were considered the three biggest warmongers in the Land of Fire.
After all, clans with bloodlines such as the Senju and Sarutobi could use Earth Release or Water Release to enhance agriculture and industry.
But those three clans could not.
The common Lightning Release and Fire Release used by the Uchiha were inherently suited for battle and had virtually nothing to do with farming. Moreover, the proud Uchiha would never lower themselves to such work.
As for the techniques of the Hyūga and Kaguya clans, they provided no benefit to civilian livelihoods. It was already fortunate if they didn't destroy production in the process.
But that was a bit off topic.
Returning to the main point.
The second reason why the major clans of the Sengoku Period rarely sent those under ten years old or below chūnin rank to the front lines was because these individuals were also valuable assets.
Even a weak genin was still a ninja, and the clan loyalty instilled from childhood made them extremely dependable.
Such people were ideal for being sent to civilian territories as local overseers, responsible for supervising tax collection and suppressing civilian unrest.
However, that didn't mean these positions were safe.
Although their material conditions might be far better than those of their clansmen fighting on the battlefield…
They also became frequent targets for raids by bandits and rogue ninjas seeking to plunder wealth and supplies.
Still, as long as their luck wasn't terrible and they didn't encounter particularly strong enemies, their abilities were usually enough to escape and retreat to one of the major cities directly controlled by the clan.
Those cities were guarded by a large number of Uchiha clansmen.
From there, they could go to the Mission Hall and post a bounty to request assistance in eliminating the threat.
For this type of mission, aside from the reward offered by the client, the clan would also provide additional subsidies.
As a result, these missions paid far better than ordinary commissions and were known within the clan as Urgent Missions.
After all, such incidents directly harmed the core interests of the Uchiha clan.
Furthermore, although the Uchiha looked down on almost everyone except the Senju, their fierce protectiveness toward their own people was practically written into their genes.
Even though the usual fate of rogue ninjas who attacked Uchiha territory was to be hunted down and killed…
In the face of huge profits, there was never a shortage of people willing to take the risk, believing they might escape the Uchiha's pursuit by luck.
So for Uchiha Kure, there was only one path to survival:
To keep becoming stronger.
If he could grow to the level of Madara and Hashirama during the later Sengoku Period, then no matter how reckless he was, it wouldn't matter.
"In the end, everything comes down to strength."
Kure narrowed his eyes slightly.
He stood up, looked around to confirm that no one else was nearby, then walked to the riverbank and gazed at his reflection in the water.
Within his blood-red eyes, two tomoe slowly rotated.
The Double-Tomoe Sharingan.
He had awakened it just last week, and it was currently the only reliable trump card he possessed.
In fact, he had first awakened the Sharingan when he was two years old, but he had kept it hidden until he revealed it last year.
No one except himself knew the exact moment his eyes had awakened.
And even now, he was still debating whether he should conceal his newly evolved Double-Tomoe Sharingan.
After all—
In this era, a ninja who didn't keep a few hidden cards up their sleeve would only be embarrassing themselves.