I brought this up in another thread.
The political theorist Carl Schmitt argued that if one consults the Greek of the Gospels, one finds that Christ's command to love ones enemies reads "diligite inimicos vestros and not "diligite hostes vestros".
In Greek inimicos signifies a private enemy, wheras hostes means a public enemy. So when Christ tells us to love our enemies, he's telling us to love our personal enemies.
This is why, as Schmitt argues, there was no contradiction in Christians waging war against the Muslims with the Gospels because the Muslims represented a public enemy to Christendom.
As Schmitt noted: "The enemy in the political sense need not be hated personally, and in the private sphere only does it make sense to love one's enemy, i.e., one's adversary."