The year 2003 was a momentous year for Iraq. The decades-long tenure of Saddam Hussein was overthrown, and the country witnessed the long-underprivileged majority rose to power, while the long-favoured minority was excluded. The year was started with US Invasion and the resultant new Iraq Government. The rise of the new government that spanned from 2003 to 2006, throughout the CPA’s reign, transition period, and the first full-term government, had an enormous impact in the marginalization of Sunni communities in Iraq. That marginalization led to Sunni resistance movement that eventually developed and produced ISIS in 2006. This research aims to explain deeper the factors of Sunni resistance movement, using the theoretical framework of constructivism, discerning how identity and ideas shaped the resistance movement and how the resistance movement ultimately developed into ISIS. The research utilizes illustrative analysis technique and qualitative approach that deeply elaborates the what about of Iraq Government 2003 to 2006 that gave rise to Sunni resistance movement and later ISIS by applying the theory of constructivism. The research concludes that marginalization during Iraq Government 2003-2006 had formed Sunnis’ identity as an underprivileged minority and further strengthened by “self” and “other” dichotomy between Sunnis and Shiites. Schism between Sunnis and Shiites was also encouraged by sectarian hatred in the middle east, particularly the role of neighboring countries, Saudi Arabia and Iran, and was also influenced by the democratization program initiated by the US. Sunni resistance movement against Iraq Government 2003-2006 was driven by Sunnis’ identity-informed interest to be included and to obtain favorable treatment from Iraq Government 2003-2006. Identity and ideas also play a vital role in explaining Sunni resistance movement as well as its development into ISIS. The shared identity and ideas between Iraqi Sunnis and ISIS towards Iraq Government 2003-2006 gave Sunnis and ISIS a common cause and common enemy in their resistance movement. Identity and Ideas ultimately united and sustained their cooperation.