The Sicilian Vespers

Peter III (second from left in upper boat) invading Sicily
Source: wikimedia commons

King Peter III of Aragon coronation as Peter I was result of Sicilian uprise against French despot Charles of Anjou. French rules in Sicily were very unpopular so when during vespers of Easter Monday, March 30th, 1282 at the Church of the Holy Spirit group of French soldiers join Italian crowd during local wedding. His misbehave cause anger eruption in local people. Angry crowd kill him and other soldiers. This bloody event start uprising that end with numerous killed French soldiers and Sicilian inhabitants.
This event gave Peter opportunity to invade Sicily to claim his rights to throne, which happened on February 2nd, 1283
Peter's wife Constance, heir of Holy Roman Emperor Frederick Barbarossa. That's make additional threads for papacy already frightened by powers of German emperors. To counterweight it pope Martin IV support French king and therefore also Charles as Sicily ruler. However Martin don't know that Peter had built his fleet with strong financial support from East Roman Emperor Michael Paleologos. Martin dream was to unite west and east church but wasn't aware about plans of his ally Charles who plan to used fleet build using pope money to conquer Constantinople although officially build for next Crusade against Islam.

Church of Holy Spirit, Palermo
Source: wikimedia commons

Successful conquering of Sicily by Spaniards led by Peter blocked whole idea of reunite either by talks (as planned by pope) or force (as planned by Charles). Next attempt was done during Renaissance at Council of Florence.

French appetite for ruling Italy returning couple of times. Remarkable is story mentioned in book about Henry IV of France who want to impress Spanish ambassador about his power: 

"King Henry IV of France  - who, as he said "ruled with weapon in hand and arse in the saddle" - boasted on Spanish ambassador of his fearsome martial capabilities. "I will breakfast in Milan, and I will dine in Rome!" roared the king ahead of a planned campaign. "Then," said the ambassador, smiling pleasantly, "Your Majesty will doubtless be in Sicily in time for vespers"." Source: Cahill, T., "Heretics and Heroes: How Renaissance Artists and Reformation Priests Created Our World", Kindle edition

Primary sources:

[1] Cahill, T. "Heretics and Heroes: How Renaissance Artists and Reformation Priests Created Our World", Kindle edition