On 25 March 2015, Saudi Arabia launched a military intervention, known as "Operation Decisive Storm", in civil-war stricken Yemen, changing the status of the conflict from 'non-international armed conflict' (NIAC) to 'international armed conflict' (IAC). Saudi Arabia conducted the operation in response to a de jure Yemeni government request, headed by President Abd Rabb Mansur Hadi, and in coordination with a coalition comprising the United Arab Emirates, Qatar, Kuwait, Bahrain, Jordan, Morocco, Sudan, Pakistan and Egypt. The United States supported the coalition by providing 'logistical and intelligence support'. The de facto Yemeni government was led by Abdul Malik al-Houthi (a Zaidi Shia Muslim) and previously, by former President Ali Abdullah Saleh (a Sunni Muslim). The de facto government was supported by Iran. Some observers described the war in Yemen as a proxy war fought on behalf of Iran and the Saudi government while the victims of the war were the poverty-stricken innocent civilians of Yemen. The conflict in Yemen raised numerous questions that this paper attempts to answer. Does Operation Decisive Storm show lawful use of force? Is Iran's support of the Houthi-led de facto government lawful? Is this conflict an international armed conflict or a non-international armed conflict? Does international humanitarian law (IHL) apply in this conflict and if so, do the parties to the conflict abide by their obligation under international law? These questions will be discussed in this paper with reference to the relevant international law provisions using primary sources as well as subsidiary sources of the law.