The World at War 1939-1945 was the first unofficial Axis & Allies expansion, published by Xeno Games.
Many of the rule changes found in the later Axis & Allies Revised & Anniversary editions (such as the 2-hit battleships, "1/2 battleship" destroyers, artillery guns) were original innovations in The World at War, first published in 1990 with 238 playing pieces and a new 24"x36" map that added more territories. As an expansion, much of the 1984 edition of Axis & Allies was needed to play this expansion.
The World at War 1939-1945 2nd Edition, published in 1993, balanced some of the gameplay and added some new ideas but retained the same glossy 24"x36" color map and pieces.
The 3rd Edition, published in 1996, continued to improve gameplay balance and added Atomic Bombs to the technology development. After developing A-bomb technology, players spend 6 IPCs to buy one, which must be delivered via a Heavy Bomber (also a required technology). Damage to a land or sea zone by an A-bomb eliminates 20 IPCs worth of enemy units and renders the territory desolate (-5 IPC value, with +1 restoration each turn for two turns), causing devastation to the economy as well as military of an opposing player.
Now in its 4th edition, The World at War includes a hardback map (replacing the poster-paper map of earlier editions), with some minor rule revisions over the 3rd ed, and this is a mature game in it's own right with differences significant enough for any A&A fan to appreciate, beyond all of the "official" editions published in recent years.
There are new unit types, such as Destoyers, Paratroopers, SS Panzers and SS Panzergrenadiers, Russian Guards, Partisans. US Marines, Rangers and Armored Infantry, U-Boat Pens (complete with the plastic figurines and/or special rules).
The rules introduce a variety of simulation fidelity enhancements, such as Banzai and Kamikaze attacks for the Japanese player, a Fall of France rule, Peacetime Production rules for the US and USSR, etc.
Axis & Allies
World At War Expansion
World at War Update 2005 Pack
Each of the seven colours should include these units which are added to an existing set of A&A Classic set:
1 Aircraft Carrier
2 Anti-aircraft Guns
1 Sub Pen