In Islam, a country that once was Muslim must always be Muslim. Spain used to be part of the Muslim empire, back in the dark ages. Not until 1492, that significant date, were the Moors finally expelled, as Christians took the province of Andalusia (what Muslims call Al-Andalus). According to this article, this still sticks in the craw of Muslims and getting Al-Andalus back features prominently in jihadist rhetoric. A former German foreign minister commented that if the jihadists fulfill their dream of eliminating Israel and taking over its once-Muslim territory, the next target would be Spain.