Both sides were equal

 Its generally believed that in the Mahabharata war,the kauravas had more warriors,and pandavas had less warriors,but still they won the war.Thats not true.Although the kaurava army has more soldiers,its the maharathis that count.They can win the war.Now Kaurava army has Bhishma,Drona,Aswahtthama,kripa,
duryodhana on its side as main dhanurdharis,which are important to win the war.The pandava side has Arjuna,Satyaki,Drishtadyumna,Bhima,Sikhandi,Abhimanyu.
Arjuna can take on Bhishma, Satyaki can fight with Ashwatthama, Drishtadyumna with Drona, shikhandi with kripa.There are also nakula sahadevas who are good arches and can fight with drona or shalya.
So both sides were equal.The one warrior who could have changed the equation was karna.With adding karna on the kaurava side,the pandava army could have been defeated within just 3,4 days.Because Bhishma,Karna make an invincible pair, both are atirathas,having learnt all divyastras under someone who was like parashurama.Drona was old,and not a kshatriya so was not able to shoot arrows with speed,accuracy.But he is still invincible with his divyastras.Bhishma knew this.Thats why he criticised karna,and removed him from battle until he is commander of kaurava army.Therefore the pandavas fought well till the tenth day when bhishma threw his weapons when shikhandi was brought in front in him.
Even if Drona was not there, Bhishma,karna would have won the war for the kauravas.
So with Bhishma,Drona and later with Drona,Karna both sides were equal.And the pandavas by killing Drona,karna by deceit won the war.
Bhishma said karna is artharatha,and he knew karna will be angry and wont fight under him.
The Pandavas before the war had approached Bhishma,Drona and aksed them as to how they could be defeated.Bhishma said when you think that you cant win the war,then come to me,I will tell how can I be defeated.Drona said if they can make him throw his weapons,then he can be defeated.As long he is holding a weapon,he cant be defeated.So the pandavas lied that Ashwatthama is dead,so that he will throw his bow.
Bhishma defeated Arjuna on the ninth day,and wrecked havoc on the pandava army.and krishna took up a chariot wheel against Bhishma.Then the pandavas appraoched Bhishma ion that night.Bhishma said bring shikhandi in front of me,I will throw my weapons and wont fight.