On Thursday, a van ran over pedestrians on Barcelona’s Las Ramblas strip. It took a very short time for ISIS to claim responsibility for the attack.

At least 13 people died and around 100 people were injured.

But why did ISIS target Spain, now, for practically the first time? 

Related: See all the terrorist attacks around the world from 2016

There could be many answers to this question.

One, could be Spain's involvement in the fight against the group in Syria and Iraq. Although Spain is not involved militarily in the fight against ISIS, it has provided logistical help and training for the Iraqi forces.

Another reason goes further back in history, according to Mia Bloom, professor of communication at Georgia State University and author of "Dying to kill: The allure of suicide terror."

She says ISIS talks a lot about the fall of Muslim rule in Spain in 1492.

"If you look at the Muslim conquest of Spain," she explains, "this was the period of the Golden Age."

She adds that for many of the Islamic fundamentalists, there are two golden ages: "There's the time of the prophets [...] and then they talk about this period of amazing expansion where Muslims got up to the gates of Vienna and they got into France."

So, Bloom says, what we see is a "hearkening back to an era which they were expanding, rather than contracting, which is what we are seeing in Syria and Iraq right now."

ISIS wants to see Spain as part of its final caliphate, like it was in the 15th century. She says that some well-known Spanish castles have been featured in the group's propaganda.

Still, this week's attack took many people by surprise, because ISIS seemed to have less power in Spain, compared to other European countries. Spain had also played such a limited role in the anti-ISIS coalition — and had largely gone unscathed previously.

From PRI's The World ©2017 PRI