According to some accounts, in the beginning of the 13th century King John of England (1167–1216) sent an embassy to the Almohad Sultan Muhammad al-Nasir (1199–1213), requesting military support and an alliance against France. At home, King John was faced with a dire situation, in which his Barons revolted against him, he had been excommunicated by the Pope, and France was threatening to invade. The embassy of three was led by Bishop Roger, and King John supposedly offered to convert to Islam and pay a tribute to al-Nasir in exchange for his help. Al-Nasir apparently dismissed the proposal.
There seems to be some skepticism whether John really made this offer and how successful it would have been if John had converted (and presumably got the rest of his kingdom to go along with this, because if he doesn't, I don't see his time on the throne being especially long). An Islamic England and Catholic Scotland and Ireland sure would give those diplomatic relations an interesting spin.
And just a little later on, over on the Continent:
Franco-Mongol relations were established in the 13th century, as Christian Crusaders and the Mongol Empire attempted to form a Franco-Mongol alliance against the Muslims.
So, the Anglo-Islamic Alliance on one side and the Franco-Mongols on the other: WHO WILL WIN???!!!!
Also posted at Dreamwidth, where there are comment(s); comment here or there.