11th century western European military challenge to Islam (AKA The First Crusade).

As Prof. William B. Stevenson wrote in The Cambridge Medieval History, it was, “The Muslim attack on southern Europe, from the eighth century to the eleventh, [which] called forth the counter-stroke which is known as the First Crusade.” (Emphasis added.) The Muslim scourge on Europe thus didn't end in the 8th century but rather intensified.

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With thanks to the extraordinary Impearls blog.