Since the Jasmine Revolution in 2011, Tunisia has been the one success story of the Arab Spring. There has been some violence, attacks on police, and political polarization; trouble along the Libyan and Algerian borders, but nothing like today's attack at the Bardo National Museum. Even previous attacks on tourist sites, such as the 2002 attack on the Ghriba Synagogue in Djerba, were outside the capital; today's attack struck at its very heart, its symbolic center of culture.
For readers unfamiliar with Tunis, the Bardo is Tunisia's national museum, and one of the world's great museums. It contains, among much else, one of the world's largest collections of Roman mosaics, which survived in the dry North African climate better than in Italy. So it is a powerful symbol of national heritage.
It is also a keystone of Tunisia's tourist industry, which took a hit after the Revolution. Tunisia's beaches are popular and affordable for European tourists, and he high profile Bardo attack is a direct attack on the industry.
But there is another powerful symbol: it stands next to the Tunisian National Assembly, recently elected, housed in an old beylical palace which shares a courtyard with the museum. So it also strikes a blow at Tunisia's nascent democracy.
In the Google image below, the complex at left (west) is the museum, the structure at the bottom (south) is the Parliament building.