lessons to be learned
Martel knew them
Coeur de Lion knew them
And more recently, Sherman, Pershing and Patton knew these things (even though only Pershing had any interaction with Islam).
Martel knew them
Coeur de Lion knew them
Don't forget Sobieski - or El Cid Campeador.
There are so many others who understood the threat, but have been largely erased by time and by the West's growing tendency to steer away from all things "Christian" when it comes to writing the history textbooks.
Take the example of King Dinis of Portugal, for instance. Not a name that is often discussed in our high schools or universities. However, he was the man who encouraged the building of a large fleet to assist in protecting against Muslim raids. When the Templars were eradicated throughout most of Europe, Dinis incorporated their Portuguese members and the wealth of the institution in the newly created Order of Christ. That order was responsible not only for the maritime defense of southern Europe against the Moors, but it was also responsible for the Age of Exploration (we've all seen pictures of the Portuguese galleons with the red cross of the Order on their sails).
Muslim raids on Morocco and Algeria forced Portugal to build a navy which, ironically, was powerful enough to explore the world and take over the Indian spice trade, thus leading the Muslim world into the economic swamp it is relegated to until this very day - despite all the oil in the Middle East.
The Age of Imperialism (and the wealth flowing in from Europe's colonies) also gave Europe the economic might to resist the Muslim onslaught. You'll never see a history teacher stitch all that together, though... and more's the pity.