Here you go again lol, He did not unite them, he suppressed them. Pretty sure the North of Iraq was not even under his control at the time of occupation.
He suppressed the Islamists, just like what Abdel Fattah el-Sisi is doing in Egypt right now. That's why the Saudis and the Qataris were against him, although he was a Sunni. And the Northern part was controlled by the Kurds, who were also secular.