St. Francis began in his early years wanting to be a Knight, and participated in some of the skirmishes between the towns of Assisi and Perugia, ending up in prison (see his youth section). During this time he did a lot of thinking, and was horrified at the reality of war. Later, beginning on a quest for the Crusades, he turned back after hearing a Voice asking him who do you want to folow, the servant or the Master? He then began his quest not for war but for Peace. Not only personal peace, but peace between fighting peoples. He spent a lot of time reconciling the fighting cities in Italy. He went to the Holy Land and went to the Crusade camps and talked with the Sultan, trying to bring peace to the two factions. There are various stories of this meeting. Here is one:

http://www.shrinesf.org/francis07.htm

In another version, in it said that he struck a deal between the
Sultan and the Christian leaders, but they did not accept it (the Christians). Some say he converted the Sultan, but that he asked to not be baptized then, as his people would not understand and would kill them both. This fascinating version can be found here:

http://www.ccel.org/ccel/ugolino/flowers.iii.xxiv.html

Because of Francis' desire for Peace, Pope John Paul II called a gathering in Assisi after the terrorist attacks on the World Trade Center. He brought together the heads of many different faiths to pray for peace. Their aim was to "confer and pray in order that world peace might yet be achieved." This powerful gathering was a tribute to St. Francis' efforts towards peace and his efforts to bring understanding between various religions. His most famous prayer is his Peace Prayer.

http://www.youtube.com/watch?v=ZI1Gst7pEqc&feature=related st. francis peace prayer by anglina filmed in assisi

Day of Prayer for Peace in Assisi:

http://www.vatican.va/news_services/liturgy/documents/ns_lit_doc_20020124_assisi-giornata_en.html

The Prayer of St. Francis

Lord, make me an instrument of your peace,
Where there is hatred, let me sow love;
where there is injury, pardon;
where there is doubt, faith;
where there is despair, hope;
where there is darkness, light;
where there is sadness, joy;

O Divine Master, grant that I may not so much seek to be consoled as to console;
to be understood as to understand;
to be loved as to love.

For it is in giving that we receive;
it is in pardoning that we are pardoned;
and it is in dying that we are born to eternal life.