It is July 1906. The Commonwealth of England invades the small Emirate of Rome, a province of the Caliphate of Arabia.
The Empire of Greater Japan comes to Arabias defence and declares war on England.
This triggers the defensive pact between England and the Caliphate of Spain, who declares war on Japan.
The Chinese Empire answers its pact with Japan and declares war on Spain. England declares war on China. This brings the Americans into the war on England's side, through their pact with Spain; and the Ottoman Caliphate as an ally of China.
Vassals of England, Brittany the Dominion of Portugal and the Dominion of Mexico all enter the war.
Vassals of Japan, the Tributary Ethiopian State enters the war.
England then declares war on the Mongol Empire. This triggers its defensive pact with the Indian Maharajate & Indias vassals the Khmer also enters the war. Spain declares war on India.
This triggers another defensive pact with the Islamic Incan Regions. England and her allies declare war on Inca.
By the end of 1906, the First World War has begun. There are two enormous power blocs:
The Commonwealth of England
Brittany
Dominion of Portugal
Dominion of Mexico
The Caliphate of Spain
The United States of America
The Caliphate of Arabia
The Emirate of Rome
The Empire of Greater Japan
The Tributary Ethiopian State
The Chinese Empire
The Ottoman Caliphate
The Mongol Empire
The Indian Maharajate
The Protectorate of Kampuchea
Note that the screenshots were taken several turns after the initial event: the foreign advisor page wasn't working, so by this point I (Inca) and Monoglia are at peace with Spain. I'm still not entirely sure what brought Monoglia & India into the war (England declared war on Monoglia in my event log), but I can only assume that they have a defensive pact with someone I don't have an embassy or contact with at the moment.
Nonetheless, this has to rank up as one of the biggest Civ conflicts: the war horn went on for a good 10 minutes! Who would have thought a some act of agression in a European backwater would lead to the first global conflict that drags in powers from distant Mongolia and South America (least of all me >.>)