In the Naruto franchise, it's safe to say Hashirama Senju, aka the First Hokage of Konoha, was the strongest of them all. He was a legend to many shinobi in the centuries that ensued, left to fear his godlike reputation. Hashirama not only put the Senju clan on the map, but he helped create peace in the ninja world by divvying up the Tailed Beasts for various nations. However, while he was the most powerful Hokage in Naruto, he wasn't the smartest.
This was clear with his handling of the Uchiha clan, namely Madara. Hashirama used diplomacy as there was so much strife back in the day, especially with Madara's people. Many soldiers were lost, precipitating a lot of hatred between both legions. Madara lost his kin too, which motivated him to want Hashirama and his younger brother, Tobirama, dead.
However, Madara would opt for peace as time passed, accepting Hashirama's treaty after many wars. Unfortunately this is where Hashirama faltered -- as he began the vision for Konoha, he left Madara unchecked. His ultimate dream was for them to rule together, but he didn't keep a proper eye on Madara or his clan -- which is what Tobirama did. Many considered Tobirama biased as he never liked the Uchihas, but he had a cruel point in that they couldn't be trusted.
Still, Hashirama was naive, believing they would change when they gave no signs of doing so. There were only red flags, which is why Madara would end up leaving and losing in a massive battle to Hashirama. This planted seeds of doubt in the Uchihas, as Madara became a martyr and infective philosophy in terms of rebellion and them getting their piece of the pie.
When Hashirama died and Tobirama took over, Madara's symbol would grow with the Uchihas feeling like they'd never be equal. It ultimately led to their terrorist behavior being quelled with Hiruzen Sarutobi in the Hokage's seat, using Itachi to kill off his clan (bar young Sasuke) and prevent a coup.
Hashirama's lack of foresight led to this civil war because had he listened to Tobirama from the start, he'd have realized he couldn't ignore blatant problems. It might have resulted in the Uchihas being exiled years before, but it'd have been way better than the eventual slaughter that awaited. After all, this genocide led Sasuke to wage war on the Hidden Leaf and almost destroy all other nations in Naruto's Kaguya War. This way, Hashirama would have known which of the Uchiha was on his side, shaping a better landscape than having the enemy secretly plotting within.
Sure, tempering hope and optimism wasn't Hashirama's style, but he needed to be harsh because with all the blood spilled between both parties, revenge was always going to be on the cards. He wasn't proactive, which is why Tobirama reamed him, and it almost backfired when Sasuke began his vendetta. Luckily, Naruto would assuage fears and unite everyone, making up for Hashirama's altruistic mistakes.