Pope Benedict XV Demands World Peace
In 1917 the First World War was raging, without showing any sure sign of concluding peacefully. By this time the Holy Father, Pope Benedict XV, who had been in the diplomatic service of the Vatican, had exhausted all of the natural means in his power to bring about peace, but to no avail. Realizing the limited power of even Papal diplomacy, the tired and frail Pope turned to the Blessed Mother of God, through Whom all graces are dispensed. He urgently requested all Christians to beg the Virgin Mary to obtain peace in the world, and to solemnly entrust the task to Her alone.
The Pope wrote of his plea for peace in a letter dated May 5, 1917:
Our earnestly pleading voice, invoking the end of the vast conflict, the suicide of civilized Europe, was then and has remained ever since unheard. Indeed, it seemed that the dark tide of hatred grew higher and wider among the belligerent nations, and drew other countries into its frightful sweep, multiplying ruin and massacre. Nevertheless Our confidence was not lessened .... Since all graces which the Author of all good deigns to grant to the poor children of Adam, by a loving design of His Divine Providence are dispensed through the hands of the most holy Virgin, we wish that the petition of Her most afflicted children, more than ever in this terrible hour, may turn with lively confidence to the august Mother of God. 
He wished the world to have recourse to the Heart of Jesus through the mediation of Mary, and he ordered that the invocation Queen of Peace, pray for us be permanently added to the Litany of Loreto. Then, confidently placing the peace of the world in Her hands, the Pope made another appeal:
To Mary, then, who is the Mother of Mercy and omnipotent by grace, let loving and devout appeal go up from every corner of the earth – from noble temples and tiniest chapels, from royal palaces and mansions of the rich as from the poorest hut – from blood-drenched plains and seas. Let it bear to Her the anguished cry of mothers and wives, the wailing of innocent little ones, the sighs of every generous heart: that Her most tender and benign solicitude may be moved and the peace we ask for be obtained for our agitated world. 
The Blessed Mother responded quickly to this agonizing plea of the Pope and the Christian people. Only eight days later, on May 13, the Virgin Mary to came to Fatima in response to the cries raised to Her from a warring world. She came to demonstrate Her maternal care for us, and that, as Mediatrix of all Graces, She alone can show us the way to peace. In order to grant this favor, though, She requires our cooperation. She requires that each one of us obey Her requests for prayer and penance, and that the Pope and bishops obey Her request for the Consecration of Russia.
May 13 was in those times the feast of Our Lady of the Blessed Sacrament! The main message of Fatima is an urgent call to repent and turn to Jesus.