The principles of international law, especially in respect of a government's interference in the contests of its neighbors, whether princes or peoples, were not, in the thirteenth century, systematically discussed and defined as they are nowadays with us; but the good sense and the moral sense of St.Louis caused him to adopt, on this point, the proper course, and no temptation, not even that of satisfying his fervent piety, drew him into any departure from it.

Distant or friendly, by turns, towards the two adversaries, according as they tried to intimidate him or win him over to them, his permanent care was to get neither the State nor the Church of France involved in the struggle between the priesthood and the empire, and to maintain the dignity of his crown and the liberties of his subjects, whilst employing his influence to make prevalent throughout Christendom a policy of justice and peace.
That was the policy required, in the thirteenth century more than ever, by the most urgent interests of entire Christendom.
She was at grips with two most formidable foes and perils.

Through the crusades she had, from the end of the eleventh century, become engaged in a deadly struggle against the Mussulmans in Asia; and in the height of this struggle, and from the heart of this same Asia, there spread, towards the middle of the thirteenth century, over Eastern Europe, in Russia, Poland, Hungary, Bohemia, and Germany, a barbarous and very nearly pagan people, the Mongol Tartars, sweeping onward like an inundation of blood, ravaging and threatening with complete destruction all the dominions which were penetrated by their hordes.

The name and description of these barbarians, the fame and dread of their devastations, ran rapidly through the whole of Christian Europe.

"What must we do in this sad plight ?" asked Queen Blanche of the king, her son.
"We must, my mother," answered Louis (with sorrowful voice, but not without divine inspiration, adds the chronicler), "we must be sustained by a heavenly consolation.