Apart from those with truly extraordinary chakra reserves, most ninja could not sustain prolonged combat. Hiruzen Sarutobi had considerable chakra, but after using the Five-Style Great Bullet Barrage Technique three times in succession, even he had been brought to the brink of exhaustion.
In this battle to defend the City, Hiruzen Sarutobi's wide-ranging Ninjutsu had been a crucial safeguard against the enemy. Yet the enemy had used their jinchūriki to eliminate that greatest threat. Without Hiruzen, the small number of Konoha and Uzumaki ninja could not hold back the Kirigakure ninja's attacks.
However, the achievements of Konoha's Research Department finally came into play. Earlier, by studying Kakuzu's corpse and analyzing Earth Grudge Fear, a secret technique from the Warring States Period, they had developed a secret technique that improved the efficiency of chakra generation, allowing ninja to recover their chakra quickly. It had now begun to see practical use—
The other achievement was the Chakra Storage Block Orochimaru had once shown everyone. Limitations in materials and technology meant that it could only store a limited amount of chakra, which also had to be extracted manually. It could not yet be used in automated ninja tools, but as a means of supplying ninja with chakra, it was still extremely useful.
Traditionally, ninja restored their chakra quickly through medicine or rest. The former harmed the body and could leave lasting side effects in severe cases, while the latter took far too much time. With these two new technologies, Konoha ninja would have far greater staying power than those of other shinobi villages.
This was the source of Hiruzen Sarutobi's confidence. He had already left a large supply of Chakra Storage Blocks in the capital. No matter how difficult the earlier battles had been, he had never used them, saving them for this