It's important to review some history to remind ourselves that the threat is real and cultural properties have already been atacked. The following is a synopsis of a deadly attack at the Uffizi Museum in Florence, Italy (source: Frommers.com):  

"Gallerie degli Uffizi (Uffizi Galleries)-- The Uffizi is one of the world's great museums, and the single best introduction to Renaissance painting, with works by Giotto, Masaccio, Paolo Uccello, Sandro Botticelli, Leonardo da Vinci, Perugino, Michelangelo, Raphael Sanzio, Titian, Caravaggio, and the list goes on. The museum is deceptively small. What looks like a small stretch of gallery space can easily gobble up half a day -- many rooms suffer the fate of containing nothing but masterpieces.

Rising from the Blast -- On May 27, 1993, a car bomb ripped through the west wing of the Uffizi, seriously damaging it and some 200 works of art and destroying three (thankfully lesser) Renaissance paintings. The bomb killed five people inside, including the museum curator and her family. While everything from a Mafia hit to a government conspiracy was blamed, the motive for the bombing, and the perpetrators, remain unknown to this day.

In December 1998, Italy unveiled what it called the New Uffizi, a $15-million renovation that repaired all damaged rooms, added more than 20,000 square feet of new museum space, and displayed more than 100 works that had never been seen before -- part of a larger project to triple exhibit space. Several branches of the book/gift shop were added to the ticketing areas on the ground floor, and the old outdoor cafe at the end of the galleries, atop the Loggia dei Lanzi with a view of the Palazzo Vecchio's tower, was reopened."

Source: Frommer's http://www.frommers.com/destinations/florence/0051020029.html