Everything you need to know.
Factions
Allies = Equestria (Or Brothers in War)
Axis = AOTS (Army Of The Sun)
Comintern = USSE
Countries
Germany = United Northern Equestria (UNE)
Britain = Pegasius
USA = Southern Democratic Equestria (SDE)
USSR = United Soviet States of Equestria (USSE)
Japan = Gryphon Kingdom
Italy = Changeling Empire
Nationalist China = Crystal Kingdom
Communist China = Crystal Republic
Poland = Denau (Pronounced Deen-Ow)
France = Prance
Russia = Almaz (Diamond Dogs)
Mexico = Marico
Norway = Normare
Finland, Ireland, Australia and Canada have the same names.
Timeline
1425: The USSE is formed by Almaz and Finland.
1436: The "Brothers in War" pact splits Equestria into separate countries including SDE, Pegasius, Prance, Ireland, Australia and Canada.
1437: Princess Celestia dies of old age.
1438: UNE's relations with Equestria sour after UNE falls into a dictatorship.
1439: UNE performs a surprise attack on Denau, and successfully takes it over in a course of three months.
1440: Equestria declares war on UNE. Not long after, the SDE head of economy, Rarity Belle, is assassinated.
1441: UNE signs a non-aggression pact with the Gryphon Kingdom.
1442: Pegasius performs a series of special forces raids on UNE. Elsewhere, GK attempts an ambitious invasion of the Crystal Kingdom.
1444 (Spring): Due to its lack of war supplies, desperate SDE infantry and armor divisions attempt to invade Marico.
1444 (Autumn): Pegasius reinforces the Prench-UNE border.
1444 (Winter): UNE, The Changeling Empire, Marico and GK join forces to form the Army of The Sun (AOTS).
1446: The Gryphon Kingdom annexes the Crystal Kingdom.
1447: The planet is plunged into darkness at the start of a world wide war. UNE annexes Prance, SDE clashes with its neighbor Marico due to the need for resources, the USSE takes over Finland, and the smell of smoke fills the air as all of Equus prepares for war.
1449: Canadian and SDE forces join arms and takeover Marico.
1452: Finnish and soviet forces are sighted on the Normarian border.
1453: USSE and AOTS sign an alliance pact, and soon enough, a number of Equestrian nations are annexed.
1456: Normare is taken over by the soviets.
1458: UNE is crushed by a very sudden USSE takeover.
1459: USSE and Equestria declare an alliance.
1460: The war ends.