The origins of the Houthi movement date to 1991, when it was created to protect Zaydism, a moderate form of Shi'ism, from the encroachment of Sunni Islamists. After the attacks on New York and Washington, DC, on September 11, 2001, the group's battle took on a geopolitical dimension, as its fighters objected to Yemen's decision to collaborate with the United States and enhance bilateral intelligence cooperation.