The propaganda of hairy-eyed Muslim fanatics such as Osama bin Laden tend to make out that the Crusades constituted an unprovoked and unjust attack on Islam by the forces of an inherently aggressive Christendom. But the way I was taught it in school the Crusades at least began as a counter-attack by Christendom against an unrelenting program of conquest that Islam had been pursuing against Christendom for centuries, and that had reached the point where it seriously threatened the destruction of the Roman (Byzantine) Empire.
Certainly the First Crusade was announced in 1095, landed in Muslim-held territory in 1097, and took Jerusalem (amidst horrifying bloodshed) at Easter 1099. While I happen to know that Islam invaded and conquered the Visigothic Kingdom in Spain in 711, and was repulsed from an invasion of France in 732 (Battle of Tours), and that Jerusalem (hitherto Christian) was conquered in 638 and Alexandria in 641, I don’t know how it started.
I guess it must have been in the time of the caliph Abu Bakr or Omar (early 630s) and of the Emperor Heraclius. Either Imperial forces attacked the Caliphate or the forces of the Caliph attacked the Empire. Which was it?
Actually a more complex question than it seems on the surface.
The hostilities between Heraclius and Muhammed were sort of simultaneous. The expansion of Islam threatened the Byzantine client kingdom of the Ghassanids, a Christian Arab state in northwestern Arabia and Syria ( similarly the Sassanians maintained their own client Arab dynasty, the Lakhshmids in northeastern Arabia ). Both the Byzantines and Sassanians saw the rise of a new centralized power in Arabia as a threat and of course Muhammed was interesting in exporting Islam to all Arabs ( which included at that time not only clientized northern Arabia, but also a substantial portion of Syria’s population ). Reportedly the first clash was occasioned by the murder of a Muslim envoy to the Ghassanids by a Byzantine vassal ( one Shurahbil ), which caused Muhammed to dispatch a force of 3,000 men in reprisal - This resulted in the battle of Mu’ta, which seems to have been on the whole a Muslim defeat, though an indecisive one ( the Muslim force made a successful fighting retreat, albeit only after the top three commanders had been killed in the fighting ). Subsequently Heraclius dispatched a counter-campaign towards Tabuk, which petered out indecisively as well. Meanwhile similar near-simultaneous ( and equally aggressive on both sides ) clashes were taking place with the Sassanians.
So, not terribly clear-cut. Probably not wise to assume particularly benevolent intentions on the part of any party.
I should probably add that it would be a mistake to regard the pre-Islamic Arabian frontier as a peaceful zone. To some extent the Caliphate inherited old antagonisms. For example one of the Arab tribes that took the lead in the attacks on Persia in the 630’s were rivals of the Lakhmids and had been the subject of a Persian punitive campaign in 606.
I’ll also note that in addition to the Ghassanids, Muhammed had sent envoys to several other rulers, including Heraclius and Khurau II of Persia, requesting their conversion :). Heraclius reporetedly politely declined, Khusrau flew into a rage and assaulted the envoy.
So in essence, hostilities grew up from the murder of a muslim diplomat by a Christian in a Christian state. Warfare between the Caliphate and the Empire began in Christian buffer state, and was well established before either side conquered any territory of the other.
In the interval from the conquest of Jerusalem in 638 to the Crimean War, what has been the longest period during which no Christian state has been at war with any muslim state?
Any Christian state with any Muslim state for any reason? Dunno - Maybe a decade or two. I’d say the immediate pre-Crimean period between 1829-1853 was pretty quiet in Europe, but you still had the first Anglo-Afghan War from 1838-1842 in the subcontinent.
But then there has probably scracely been a year when some one was not actively fighting someone, somewhere.