ISIS has been fighting for many years in the past two decades. In order to do that much fighting, they need to have money for soldiers and supplies. Fighters need weapons, ammunition and tanks to fight the enemy because they are fighting against countries that have their own armies. When ISIS began in 1999, it was still a militia and therefore it had to find its own money.
During the two decades that ISIS has been in existence, they have received money from surrounding countries who have either given them money out of support or fear. Some of these countries include Qatar and Saudi Arabia. However, laws have been enacted in some of these countries to stop the support of ISIS.