Though the immediate cause of the war was a rapid escalation of sectarian conflicts and civil wars in the Middle East, the long term causes of the war were much deeper. A cold war type conflict for world influence had been long fought between US and its NATO allies and Russia and China, and to some extent India. Additionally, Shiite Iran and Sunni Saudi Arabia had long been vying for leadership of the Muslim world. There was deep mistrust among both sides, and each was unwilling to let its proxy faction lose ground, lest control of the vital resources in the region slip away from them completely. This led to the startlingly rapid escalation of what would otherwise be fairly common civil wars in the Middle East.
Start of the War
As Arab Spring took hold of North Africa and the Middle East, Saudi Arabia and its Sunni allies in the Gulf states began funding secular and Islamist rebels against the Assad Regime in Syria, Mubarak in Egypt, and Qaddafi in Libya. Iran responded by supporting Shiite groups in these regions. Iran also supported the Houthi rebels in northern Yemen. The KSA and its allies responded by supporting the Yemeni government with air strikes and ground support. In turn, Iran created and funded the Qatari Workers Liberation Army to destabilize the Saudi ally. Saudi Arabia launched a coalition invasion to remove the Qatari rebels. Pakistani troops in the coalition, ended up killing 139 Indian laborers, sparking an Indian invasion of Pakistan. The coalition declared war on India, subsequently Iran, Russia, and China declared war against the GCC coalition. The United States and NATO then declared war. After that point, almost every major world power had entered the war, causing many people to call it World War 3.