ShriVishnu knows everything, yet He lets Pandavas and Karna be at fight. Moreover, Karna, born to ShriSurya, the Illuminator of whole Universe, had to suffer worse than the most unfortunate mortal.
Krishna tried to.
Sama, Dana, Bheda, Danda - neeti - are the 4 types of bringing about reconciliation between two enemies, to be followed in that order preferably.
When Duryodhana said he wouldn't return Pandavas kingdom after 13 years as per agreement during Dice game, Pandavas send Krishna as messenger to Kauravas to talk samadhaan (diplomatic dialogue). When Duryodhana refuses, Krishna asks him to gift (Dana) 5 cities, or 5 villages, or 5 houses, or at least a house with 5 rooms. He still refuses.
Now, Krishna knows Duryodhana is ready to go to war because he feels he can rely on Karna as his biggest strength/weapon. So, Krishna goes to Karna, tells him the true story of his birth, that his mother is Kunti, and Pandavas are his younger brothers, and asks him to join his side (Bheda)
Karna however says he is 'indebted' to Duryodhana since he gave him Anga desh as gift during Pandavas 'graduation' ceremony where Karna was insulted that he could not participate in Dhanur Vidya exhibition/competition, as he was charioteer's son.
So, he refuses to join Pandavas' side.
So finally, Krishna takes up punishment (Danda), and orchestrates Kurukshetra war.
Also, just because two people are brothers does not mean they should not fight if one of them is doing Adharma. In previous Avatara, Shri Rama showed this twice - Vali vs. Sugriva, Vibhishana vs. Ravana. He didn't tell Sugrive that he should not fight with his brother Vali, or Visbhishana that he should not fight with his brother, Ravana.. because Rama always takes side of Dharma, irrespective of being one's brother or any other relation.
We all have free will.
God can show us the path, but if we refuse to walk the path, he won't force us to.
Sri Krishna's main task was total annihilation of the Kshatriya race that was predestined towards the end of Dwapara and beginning of Kaliyuga. Kshatriya race wielded so much power that Krishna did not want to leave them to misuse that power in kalyuga. The later Kshatriays were conceived with brahmana men and Kshatriya women and were not as powerful as the ones that were annihilated. In Stree Parva of Mahabharata of Veda Vyasa, Gandhari is distraught after viewing the dead bodies in Kurukshetra after the battle. In anger and grief over the death of her sons and the Kaurava soldiers, Gandhari curses Krishna with the death of all Yadavas in a manner similar to the death of her sons. She blames Krishna for his inaction and believes that he could have prevented the war and the slaughter of hundreds of millions people who died in the war. Krishna's answer is interesting. This is from Mahabharata, Book 11, Stree Vilapa Parva, Section 25 (translation by Kisari Mohan Ganguli): "Hearing these words, the high-souled Vasudeva, addressing the venerable Gandhari, said unto her these words, with a faint smile, ‘There is none in the world, save myself, that is capable of exterminating the Vrishnis. I know this well. I am endeavouring to bring it about. In uttering this curse, O thou of excellent vows, thou hast aided me in the accomplishment of that task. The Vrishnis are incapable of being slain by others, be they human beings or gods or Danavas. The Yadavas, therefore shall fall by one another’s hand.’ After he of Dasharha’s race had said these words, the Pandavas became stupefied.