After developing the Flying Thunder God Technique, he had used it on the battlefield for the first time and directly killed Madara's younger brother, Izuna Uchiha.
One had to know that before then, Izuna had always suppressed him in terms of strength. After all, Izuna had already awakened the Mangekyō Sharingan at the time, and his strength surpassed his own.
Yet even so, he had still killed him with the Flying Thunder God.
However, while killing Izuna, he had also created an even more formidable enemy for the Senju Clan—Madara Uchiha.
The Second Hokage remembered that when Madara learned Izuna had been killed by him, he had flown into a rage.
He knew very well that if not for the First Hokage stopping him, Madara might have killed him long ago.
After Izuna's death, Madara transplanted his eyes, allowing his Sharingan, which had been on the verge of blindness, to evolve. He gained a pair of Eternal Mangekyō Sharingan that would never lose their light.
The first thing Madara did after obtaining the Eternal Mangekyō Sharingan was seek revenge—revenge against the Senju Clan.
At the time, he was brimming with confidence. He appeared in a new form and launched a ferocious assault directly against the Senju Clan's territory.
Back then, Madara believed that no one could stop him anymore, not even Hashirama, who had once been evenly matched with him.
But the final result proved that he was wrong. Even after unleashing his complete-body Susanoo, he was still suppressed by Hashirama.
It was at that moment that Madara realized Hashirama had been going easy on him throughout their previous battles.
His proud self-esteem had been wounded.
After being defeated by Hashirama that time, Madara lost his will to fight. Then, after Hashirama's various attempts at persuasion, he led the Uchiha