An objective look at Mahabharata reveals the real reasons behind the war.
Some people ask, "What was the reason for the Mahabhilrata war? Wasn't peace a better solution? Or perhaps," they present an afterthought, "it was due to the corrupt government of Duryodhana." 
According to the scriptures, Duryodhana went to heaven for his kingly duo ties. You cannot say he was completely a good person, but as a king, according to karma-kanda, the devas were quite pleased. 
The main reason I know why they fought is that all the devils asked the supreme Lord Visnu to descend and destroy the race of asuras who inh abited the earth – asuras like Jarasandha and others who Krsna killed. These asuras had big plans for Duryodhana and wished him to rule the world. The greatest devas – Vayu, Indra, Yama, and the Asvinis had descended as the sons of Pandu in order to fight in the war and were led by Krsna Himself. A similar scenario had occurred at the battle on the milk ocean – Visnu and the devas fought aga inst the asuras. Indra and Vayu did a lot of fighting. However, Visnu appeared to help them when they needed Him the most. In the case of Mahabharata , on the earth, Arjuna and Bhima did most of the fighting and Krsna saved them when they needed Him the most. 
Vaisampayana continued, "Brahma then commanded all the gods saying, 'To ease the earth of her burden,go and take birth there and according to your respective roles, seek war with the asuras already born there.' All the gods including Indra, on hearing these words of the Lord of the celestials, accepted them. And they all having resolved to come down on earth in their respective roles, then went to Vaikuntha to meet Lord Narayana, the slayer of all foes. Indra, the most exalted of persons, addressed Him, "Please incarnate." And Lord Hari replied, "Let it be." Then [ndra had a consultation with Narayana about the latters descent on the earth from heaven with all the gods according to their respective roles. Then, having commanded all the dwellers in heaven, Indra returned from the abode of Narayana. The dwellers in heaven gradually incarnated on earth for the destruction of the asuras and for the welfare of the three worlds. And then, O tiger among kings, the celestials had their births, accordingly as they pleased." 
Based on scriptures, this is the 'Why' answer to the initial question. 
The following is the 'how' answer. 

Lord Krsna Comes To The Rescue Of Draupadi

When the Pandavas were cheated out of their kingdom by Duryodhana, Dusasana grabbed Draupadi by the hair and dragged her into the assembly. She was wearing only one very light cloth and was in no condition to be seen in public. However, Dusasana took her, a chaste woman, into the Kuru hall. 
She begged all the Kuru seers to tell her why she was being treated in this way. Dusasana grabbed her by the hair and pulled very hard saying, "Because you are a SLAVE! SLAVE! SLAVE!" Then he threw her on the floor. Karna added, "How funny it is that you ask this question! You are a prostitute and do not even deserve to walk around with clothes. Dusasana, strip her naked." 
(And I wonder why people say Kama is their hero.) 
Duryodhana pulled back his lower cloth, showed her his thigh, and said, "Let the lady sit here." Dusasana tried to strip her naked. However, Krsna saved her. 
According to the ksatriya custom, the way Draupadi was treated by the Kurus was enough grounds for warfare. 
Bhima made a vow to rip the heart out of Dusasana, break the thigh of Duryodhana and kill all the 100 sons of Dhrtarastra. 
This incident shows how seeds of the war were sown. 
In this modern age, people think they know everything and that they have the solution for the problems of the world. People want peace and when they hear stories like Mahabharata then they think it is promoting warfare. Idealism is a wonderful thing, but reality is even more wonderful, and Mahabharata is a book about just that. 
I ask these people, "What would be your response if what happened to Draupadi were to happen to your wife?" The fact that the Pandavas went into exile and even tried to have a peaceful solution is testament to their character. A ksatriya's nature is to fight and legally by their code of honor, the Pandavas had every right to engage in warfare with the Kurus. 
Idealism is good but this world is anything but ideal. 
As Krsna says to Sainjaya, who was asking that the Pandavas make peace – Udyoga Parva : 
"The gods have declared: Philosophers may speak about the glory of food for hours at a time, but can mere speech fill their belly? What is the use of this idealism? You speakof peace, O Sanjaya, but did virtue come to your lips when the Pandavas were being cheated in the game of dice? What lessons in morality did you give to Dhrtarastra when Draupadi was dragged into the assembly?" 
Sometimes people think that Mahabharata is a mythology meant to give morals. An honest reading of the book will clearly reveal that this isn't the case. It is a history book and it is full of facts life in this world is made of – politics, betrayal, diplomacy, espionage, truthfulness, loyalty, character, and ultimately, devotion to God. 
The root cause of doubt in ancient scriptural wisdom is faithlessness. People don't understand the how's and the why's and then they point their finger at Krsna. 
"Why should there be a war?" 
What they are really asking is, "Why is the world so bad?" The answer is – because of our ignorance and non-adherence of the will of God. 
If we surrender to the Lord we can live a wonderful life and return to him, but that requires a change of consciousness. Many great authorities say, and I can testify myself, that chanting the Lord's holy names, surrendering to Him, and having faith in Him will give us the proper vision of things. It will help us to live a very sublime life despite all setbacks. 
This is one very important lesson that we can learn from the lives of the Pandavas.  
Bhakta Jagannatha is a second-generation ISKCON devotee. He lives in New Vrndavana, USA and is well known for his wondehul recitation of Mahabharata .