When Spain was beginning to re-take their country back from the Muslims in the 1100s, both the Catholics and the Muslims engaged in philosophical discussion. It was upon those talks that the two religions actually had some sort of common ground. This was all outside any kind of military problems.
Around the time that the Toledo translators were working on Aristotle's Ethics which the West had not seen, the top imams decided that Islam would no longer allow any Muslim to study philosophy. They actually put a fatwa out on anyone caught reading philosophy. They could not even read the commentaries that their own muslim philosophers had written.
That fatwa put the cabosh on any sort of dialogue with the West. And since that time their has been no philosophical exchange with the West. The Muslim world is actually stuck in the Middle Ages intellectually.
What Pope Francis has done is work to open this dialogue back up. I see the merit in it, because without philosophical dialogue there is no greasing of tensions. And Lord knows, the people most interested in true peace should be free to speak to each other.
No. It was the defeat of Islamic forces that led to their retreat. There is no common ground Islam. They are commanded to kill, convert, or subdue "the people of the Book"