Japan's imperialistic ambitions to dominate the entire Pacific may seem a mad gamble when viewed from a comfortable 21st Century perspective, but it should be remembered that, in the 1940's, empires still existed. European powers had commanded vast tracts of the Pacific region for centuries, and the British Empire remained the largest in history. On the other side of the world, Germany had dominated almost the whole of continental Europe, so the idea of powerful nations taking territory by force was an attractive idea that, today, might seem repugnant.
Despite naval actions in oceans over the globe, it was Japan's entry into the war that truly turned hostilities into a World War. By attacking Pearl Harbour, Japan ensured the United States was put into a position whereby it could no longer remain neutral. No longer was the fighting confined to Europe and Africa, with warships trading blows overseas. Now the people of almost every continent were directly affected by war.
Ultimately, Japan failed in its gamble. Before it fell, however, it had dealt the United States a grievous blow, and all but ended the hold European countries had in the region (not least because they had their own problems back in Europe). Once forced on the defensive, Japan proved to be a terrible enemy, its people willing to fight to their last breath, ferociously resisting the advance of the Allies as its forces were gradually pushed back to the Japanese islands.
The end of the Pacific war marked the end of the Second World War as a whole, an ending that was mushroom-shaped and terrifying., leading on to a new struggle that would hold the world in its grip for the rest of the 20th Century.
This supplement for Battlefield Evolution: World at War contains army lists and new rules for the following armies, in all periods of the Pacific War.
British Army and Commonwealth
Imperial Japanese Army
Imperial Japanese SNLF
United States Marines Corps