One of the single most important factors in ISIS’s resurgence is the conflict between Iraq’s largest two Arab religious groups: Shias and Sunnis. ISIS fighters themselves are Sunnis, and the tension between the two groups is a powerful recruiting tool for ISIS.
In the most basic theological terms, the Sunni-Shia split in Islam originated with a controversy over who would take power after the Prophet Mohammed’s death. Today, of course, Iraq’s sectarian problems aren’t about relitigating seventh-century disputes; they’re about modern political power and grievances. But those do tend to fall along Sunni-Shia lines.
A majority of Iraqi Arabs are Shias, but Sunnis ran the show when Saddam Hussein, himself Sunni, ruled Iraq. Saddam spread a false belief, still surprisingly persistent in the country today, that Sunnis were the real majority in Iraq. Thus, Sunnis felt, and still feel, entitled to larger shares of political power than might perhaps be warranted by their size.
Meanwhile, the Iraqi civil war sparked after the 2003 US-led invasion had a brutally sectarian cast to it, and the pseudo-democracy that emerged afterward empowered the Shia majority (with some heavy-handed help from Washington) at the expense of the Sunni minority. Today the two groups don’t trust one another and so far have competed in what they see as a zero-sum game for control over Iraqi political institutions. In 2013, Shias used control over the police force to arbitrarily detain Sunni protestors demanding more representation in government.
So long as Shias control the government, and Sunnis don’t feel that they’re fairly represented, ISIS has an audience for its radical Sunni message. That’s an important part of how the group built up support in Iraq’s heavily Sunni northwest.