Senodyoga Parva is the forty ninth upa parva included in the fifth maha parva, namely the Udyoga parva in Mahabharata. Vyshampayana had narrated the historic events to Janamejaya on his queries.
How Krishna showed his desire was peace, after the marriage of Uttara?
After the marriage of Uttara, on the fourth day, there was a court session in the capital city of Matsya. Distinguished royal guests attended the session, mainly the close family relatives of Pandava brothers, and the relatives of King Virata. The kings were allowed in the beginning to chat with one another and later they had occupied their eligible seats as fit to their ranks before king Virata.
The issues of the local people were discussed initially and disposed early, and later, they turned into the subject of the provision of the lost kingdom of Yudhishtira to back to him. Krishna and Balarama were seated on either sides of their father and the entire people in the court session silently and eagerly looked towards Krishna.
Krishna finally started speaking and advised to send a suitable messenger to Nagasahvaya of Duryodhana to decide for re-posting Yudhishtira as the king of his lost land, by re-granting him half of his ruling area. He announced both the parties, Pandavas or Kauravas are equally related to him and later, he left to Dwaraka after wishing a happy future life for the newly wedded Uttara family.
Why Yudhishtira gathered trained soldiers in Upaplava?
Yudhishtira with his brothers and Draupadi lived in Upaplava camp that was provided on a temporary settlement basis. Many kings provided them with many presents and assured help in times of need, if any. Since he was then a relative of Virata, father in law of princess Uttara, it was a necessity of that time to keep royal status, and hence to have soldiers for defense also.
In the later days, Yudhishtira being obliged by Virata, forced to gather trained soldiers, since in times of any attack to Matsya, had to provide over all military protection to him, as Kichaka or Upakichakas, the former army commanders were no more then. He got the support of seven thousand akshohini trained soldiers.
From Panchala, his brother in law, Drupada came to his help. From Madra nagara, Shalya was there to sympathise with him. The sons of Pandavas and Abhimanyu were able warriors, fully trained. The primary requirement of the Pandavas was to provide military help to Virata of Matsya.