Hearing Fugaku's words, Sasuke, who had been wearing an "angry" expression, froze on the spot. This was completely different from the outcome he had expected from the start!
When they invaded Konoha, Sasuke had indeed followed Uchiha Fugaku's orders and immediately sought out Rin, the Three-Tails Jinchuriki, fighting her—the very scene Sakura had witnessed.
However, not long after the battle broke out, Sasuke, who had fallen into the ruins with Rin, called off the fight and revealed his identity as a double agent.
When they learned that Sasuke had remained beside Uchiha Fugaku as an undercover agent in order to take revenge, and when he produced Yoru Machi's token as proof, Rin, Obito, and the others finally believed him.
While Fugaku was fighting Naruto and the others, Sasuke told Rin, Obito, and the rest everything he knew.
Upon learning that the Uchiha Clan had been incited from within by Uchiha Fugaku and had ultimately been wiped out, Uchiha Obito was also furious.
To be fair, he did not have an especially strong sense of belonging toward the Uchiha Clan. But Obito, before his fall into darkness, had always been an Uchiha with a powerful sense of justice. Given his nature, he simply could not accept Fugaku sacrificing his clansmen, even his blood relatives, for his own sake!
To gain more trust and create a better opportunity to strike, Sasuke and Rin had put on an act and brought back one of the Three-Tails' tails—one convincing enough to pass for the real thing.
Yet Sasuke had never expected that this single tail they brought back would actually fulfill the conditions for the Ten-Tails' revival...
Because the Ten-Tails had revived earlier than in the original storyline, Uchiha Fugaku's side gave the Shinobi Alliance no time to prepare for this Ninja World War.