Sahadev knew about the Mahabharat War

Sahadeva was a great astrologer and was supposed to have known the events of the Mahabharata War beforehand but was cursed that if he disclosed the knowledge, his head would split in pieces. Hence, his relatively silent role in the epic compared to the other brothers. The Bhagavata Purana has a passage in which Sahadeva predicts events of future when asked by his elder brother Yudhisthira, the king.
Of the five Pandavas, Sahadeva was the youngest. He is nevertheless referred to as the wisest of all of them. Yudhishtra even speaks of him as wiser than Brihaspati, the divine teacher of the Devas. It is also believed that Sahadeva was an incarnation of the great planet, Sukracharaya, the Asura Guru. It is therefore not surprising to note that he was one of the few contemporaneous persons living with Lord Krishna, like Bhishma and Vidura, to realise that Lord Krishna was the almighty Para Brahman Himself. Lord Sukra is considered smarter than Brishapathi, as he would always discover and identify Lord Sriman Narayana, even when the Lord was in disguise, as it is his duty to protect Asuras. The true greatness of Sahadeva lies in the fact that it was he who performed Agrapuja to Lord Krishna, declaring openly amongst Kings, in the face of opposition, that Lord Krishna, being Parabrahma Swarupi, deserves the first respect.
As per the Bhagavata Purana, he is one of the greatest devotees of Lord Krishna. Lord Krishna once asked Sahadeva, what should be done to stop the war. Sahadeva told him that Krishna must be tied down and imprisoned and all the Pandavas along with Duryodhana must be sent to forest and Karna must be made the king. When Krishna challenged him to tie him down, Sahadeva started meditating and envisioned Krishna as a small baby and tied him down. Since Krishna could not move out of his bondage created by Sahadeva in his meditative trance, he blessed him with divine vision and Sahadeva released Krishna from the bondage.
At the time of Mahabharata War, both the sides were trying to give sacrifice to Goddess Durga seeking success. Duryodhana sought the most auspicious time from Sahadeva for performing the sacrifice. It is believed, in Hindu school of thought, that the time of sacrifice, denotes the commencement of war and thus decides the very outcome of the great war. Sahadeva advised him to perform the sacrifice on the new moon day to ensure success in war. When questioned by his brothers and Lord Krishna, Sahadeva gave the polite reply that he is ready to die doing his swadharama of being an astrologer. Probably, he was following the teaching of Lord Krishna even before the Bhagavad Gita was given out. Lord Krishna was supposed to have played a trick on the Sun and Moon gods to create the Bodhayana Amavasya which occurs one day earlier than the regulation Amavasya. The Pandavas performed the sacrifice on Bodhayana Amavasya day and obtained the boon of success from Goddess Durga