While the Aztec Hegemony and Incan Empire had already been at war for nearly two decades, and the Aztecs were invading the terriotires of the Comanche Empire and Plains Confederacy, the war was said to have officially begun with the combined attacks by the United Islamic Republic, Kingdom of Ghana, and Free African Union against the Egyptian Empire. The three powers agreed to divide Egyptian territory among themselves, forming the Grand Powers. The Empire of Japan, longing to be on the same level of its Chinese rivals, also joined the Grand Powers, and with the UIR, participated in an invasion of China. An appeal was secretly made to both Azteca and Inca, and while the Incans refused, the Aztecs accepted, completing the Grand Powers. With Ghanese and UIR support, the Aztecs managed to defeat both the Comanches, and the Tribal Alliance, conquering them. A combination of Ghanese attacks on Roman North Africa, and UIR-Japan combined attacks on Russia, brought the Romans and Russians into the war as well. The last country to get involved was the Norse Kingdom, until the King was pressured into joining the Coalition by the Althing.
The advance of the Grand Powers came to an end with the defeat of the Ghanese by the Egyptians and Romans at the Battle of Cyrene, and the defeat of the Japanese by the Chinese at the Battle of Taiping. Coupled with the victory of the UIR forces at Mt.Sinai, which broke the UIR's advance, Egypt began to push back, taking back much of its lost territories, since taken by the Grand Powers. As Egypt advanced rapidly against the Ghanese, the Free African Union dropped out of the Grand Powers, and sued for peace with the Egyptians. No longer preoccupied with the FAU, the Egyptian Imperial Army pushed back against the Ghanese and Islamists.
With the formation of the United Islamic Republic, the geopolitical world changed significantly. Having been formed out of a total five nations, all of them Islamist in nature, the trade and economies of the world were impacted significantly. With the UIR controlling all oil reserves, alternate sources of power were sought out, and the oil price fell steadily; this had a drastic effect on the UIR economy, causing it to also decline. The extremist Islamist movements were on the rise, with President Khalid Sayed rallying many behind the Islamist movement, calling for Islamic domination over the region, and for Egypt to be expelled completely.
In the Americas, the Aztecs and the Incans were already in an extended war with each other, in a competition over the northern regions of South America. As the Grand Powers formed, both sides were offered membership (unbeknownst to the other), and assistance in defeating their enemy. The Incans, not wanting to risk conflict with the Egyptians, whom they viewed as a greater threat, refused. The Aztecs, however, tired from not just the war with Inca, but also with the Comanches and Plains people in the North, agreed.
While the exact cause of the war is in dispute, it has been theorized that the first sign of war was the assassination of Horemheb XII by Mahmoud Abbas. While the UIR denied involvement, the Egyptians found what they believed to be sufficient evidence against them.