When the teams for the Mahabharata war were bring decided, Lord Krishna had told both the teams that they had to choose between Krishna and the entire army. When the condition was presented before the Kouravas, they chose the entire army and left Lord Krishna to the Pandavas.
However, the Pandavas knew that Lord Krishna was no less than a divine soul. They were happy about the fact that Lord Krishna was on their side.
Lord Krishna actually led the Pandavas to victory. But the question is how he did so. Well, Krishna was very a sharp and intellectual. He used such wonderful plans that the battle turned towards Pandavas' victory. Every time the Pandavas seemed to lose the battle, Lord Krishna would come up with a plan to save them.
When Bhishma Pitamah seemed heavy on the Pandavas, Lord Krishna brought Shikhandi in the battle. Shikhandi had an enmity with Bhishma Pitamah since the past life. Shikhandi was born neither as a complete man, nor a woman. Krishna invited him as a man to the battle. But Bhishma Pitamah could not attack her as she was half-woman.
This tact of Krishna helped the Pandavas and thus Bhishma Pitamah fell weak. Krishna then persuaded Arjun to attack Bhishma Pitamah with arrows. Bhishma Pitamah thus slept on the bed of arrows.
Dronacharya was another person who could have made it difficult for the Pandavas to win the battle. Krishna persuaded Yudhishthira to speak the truth in such a way, that Dronacharya would misunderstand it. Yudhishthira was a righteous man, who would never tell a lie. There was an elephant in the battle with the same name as that of the son of Dronacharya, Ashwatthama.
When Yudhishthira was answering Drona's questions saying that Ashwatthama was dead, Krishna blew the conch. Yudhishthira had completed only half the sentence by then, which also said Ashwatthama was an elephant and not Drona's son. Dronacharya misunderstood and thought his son was dead. Just when Dronacharya dropped the arms, Dhrishtadyumna from the Pandava team, attacked him to death.
Arjuna had vowed that he would kill himself if he was unsuccessful in killing Jayadratha before the sunset. When Jayadratha got to know about this, he went into hiding and waited for the sun to set. Krishna soon directed his Sudarshan chakra towards the sun, so that all misunderstood that the sun had set.
Thinking that sunset had happened, Jayadratha came out demanded that Arjun kill himself as per the vow. But just then Lord Krishna removed the Sudarshan chakra and the sun came out. As soon as he realized that the Sun had not set till then, Arjun killed Jayadratha.
Lord Krishna knew that the lower part of Duryodhana's body was weaker than the upper part. Lord Krishna advised Bhima to attack him on the lower part of the body. Bhima did as directed and thus killed Duryodhana, one of the bravest and the strongest members of the Kourava team.