The same sultan submitted this problem to [Francis]: “Your Lord taught in his gospels that evil must not be repaid with evil, that you should not refuse your cloak to anyone who wants to take your tunic, etc. (Mt 5,40): All the more Christians should not invade our land!”. 

And Blessed Francis answered: “It seems to me that you have not read the gospel of our Lord Jesus Christ in its entirety. In fact it says elsewhere: “if your eye causes you sin, tear it out and throw it away” (Mt 5 , 29). With this, Jesus wanted to teach us that if any person, even a friend or a relative of ours, and even if he is dear to us as the apple of our eye, we should be willing to repulse him, to weed him out if he sought to take us away from the faith and love of our God. This is precisely why Christians are acting according to justice when they invade the lands you inhabit and fight against you, for you blaspheme the name of Christ and strive to turn away from his worship as many people as you can. But if you were to recognize, confess, and worship the Creator and Redeemer, Christians would love you as themselves instead.”
–Sorry, don’t have the source handy