Sage Mode was powerful. That was only natural.
In the original story, Jiraiya had been able to battle the Six Paths of Pain using Sage Mode, even gaining the upper hand at one point during the fight.
So using Sage Mode against the Uchiha brothers, who possessed the Mangekyō Sharingan, was indeed the best choice.
But after seeing Uchiha Itachi's reaction, Jiraiya knew this battle probably would not happen.
He had already noticed that Uchiha Itachi was not under Kabuto Yakushi's complete control at the moment.
That meant that even if they did fight, his opponent was bound not to go all out.
Since that was the case, he had no need to enter Sage Mode.
After all, the war had only just begun, and many more battles awaited him. Every bit of strength he saved now would give him that much more power to help the Allied Shinobi Forces later.
So Jiraiya abandoned his attempt to enter Sage Mode and, like Uchiha Itachi, began waiting for the outcome of Tobirama Senju and Sasuke Uchiha's battle.
But while those two were locked in combat, Uchiha Itachi suddenly stirred. He abruptly pulled his gaze away from the battlefield and looked at Jiraiya instead.
Kabuto Yakushi was urging him on.
It seemed he could no longer keep watching.
The next second, Uchiha Itachi flung several shuriken at Jiraiya.
Spinning rapidly through the air, the shuriken pierced the air and streaked down like meteors, too fast for the naked eye to follow.
"Clink, clink, clink..."
Several of the spinning projectiles collided, causing the shuriken to abruptly change direction. They traced arcs through the air and continued descending toward Jiraiya.
Although Uchiha Itachi had thrown only a few projectiles, their midair changes in direction allowed the shuriken to completely seal off every direction around