The entire Shanthi Parva can be read here.
Even Krishna broke his own vow so that Bhishma could keep his.
Lord Ram gave up his kingdom for 14 years.
Bhishma gave up his chance of marriage and having children and his entire right to the kingdom that was rightfully his.
Bhishma always thought that he followed Dharma.
But he did not follow the advice of Krishna who advised that Dharma should be followed at all costs, even if you have to break your vow or kill your own wicked family.
Bhishma also was doing what he thought was his duty to defend his king, even though the wickedness of the kings sons brought about the war.
His failure to truly follow Dharma lead to a destructive war and he ended up lying on a bed of arrows which pierced every inch of his body.
The Shanthi Parva underlines the futility violating Dharma and futility of hatred, war and the great loss caused to families and nations by fighting over land.
As Bhisma said, fighting over land is like fighting over a mother by her children.
Can a mother ever be divided among her children, she belongs to all her children and seeing her children fight causes her great suffering.
In the same way Land(Mother Earth) belongs to all so it is foolish to claim it solely for oneself and fight over it.
The Shanthi Parva also stresses on the importance of upholding the Dharma and importance of dedication to Dharma and then the Nation, regardless of relationship and not being blind to the flaws of ones own family and children when they violate Dharma and put the Nation in danger.
All those who did not uphold this basic fact finally suffered great losses to the bad actions of the very families and children whose flaws they were blind to.
Dhritarashtra and Gandhari lost all their sons and also their kingdom and Bhishma ended up lying on a bed of arrows because they were blind to and accepted the faults of the Dhartarashtra(sons of Dhritarashtra).
Yudhishthira was heartbroken at the loss.
He then said in Section VII:
Fie on the usages of Kshatriyas, fie on might and valour, and fie on wrath, since through these such a calamity hath overtaken us.
Blessed are forgiveness, and self-restraint, and purity, with renunciation and humility, and abstention from injury, and truthfulness of speech on all occasions, which are all practised by forest-recluses.
Beholding those kinsmen of ours that were bent on acquiring the sovereignty of the world slain on the field of battle, such grief hath been ours that one cannot gladden us by giving the sovereignty of even the three worlds.
Alas, having slain, for the sake of the earth, such lords of earth as deserved not to be slain by us, we are bearing the weight of existence, deprived of friends and bereft of the very objects of life.
Yudhishthira was being very idealistic.
Poverty is a state of sinfulness. It behoveth thee not to applaud poverty, therefore. The man that is fallen, O king, grieveth, as also he that is poor. I do not see the difference between a fallen man and a poor man. All kinds of meritorious acts flow from the possession of great wealth like a mountain.
When, the very gods have won their prosperity through internecine quarrels, what fault can there be in such quarrels? The gods, thou seest, act in this way. The eternal precepts of the Vedas also sanction it.
We never see wealth that has been earned without doing some injury to others.
TO BE CONTINUED