Here are some of my thoughts on some of the major/minor nations.
A lot of these are drawn from Imp Game’s The Great War, so I will reference that game a lot.
Italy: had a defensive treaty with the Central Powers/Triple Alliance but went over to the Allies/Entente when the CP turned out to be the aggressors in the war. Spent most of the war fighting the Austrians and eventually gained some territory from them. I would like to see Italy as an “Entente†power, possibly with some special rules, and have some optional rules for Italy entering the war on either side.
TGW status: Major Neutral (mostly to preserve the 3v3 setup of the game, IMHO), supporting either Germany or France
USA: tipped the scales of war against the Germans; by that time, the Ottoman and Austro-Hungarian Empires were in collapse, and the outcome was basically a foregone conclusion, despite the Brest-Litovsk Treaty which granted Germany a great deal of territory in the east (later undone by the Treaty of Versailles)
TGW status: Major Neutral, represented only by convoy zones, supporting the UK
Bulgaria: History identifies Bulgaria as the 4th of 4 Central Powers, after Germany, Austria-Hungary, and the Ottoman Empire. In TGW, it is often more tactically useful (so i am told) for the CP to invade Bulgaria, even if they are supporting the CP. Bulgaria and the Ottomans actually fought on opposite sides of several Balkan wars in the 1800s; as I recall, the alliances against the Turks often broke down into self-interests and backstabbing between Bulgaria, Serbia, and Greece.
TGW status: Major Neutral, supporting either the Ottoman Empire or Russia
Serbia: historically the fighting between Austria and Serbia went on for a long time; in TGW it is usually over by Turn 1. The rules of TGW state that if attacked and not conquered, Serbia joins Russia; I always thought it should just be a Russian territory from the get-go to avoid the need for this special ruling.
TGW status: Minor Neutral (special)
The Balkans: From my understanding of history, Greece was pro-UK, Montenegro was pro-Serbia, and Romania was pro-Russian (or at least anti-Hungarian).
TGW status: Minor Neutrals; can be influenced by Austria, Ottomans, or Russia (Imperious Leader can probably correct me if I am misremembering this)
Scandinavia: Norway, Sweden, and Denmark were neutral in WWI, and were not invaded (unlike WWII where Denmark and Norway were invaded by the Germans, and Sweden supplied Germany with iron ore). Also of note, Finland was under the control of Tsarist Russia during this era. I feel like Sweden and Norway could be left off the map in A&A:1914
TGW status: Minor Neutrals; can be influenced by UK, Germany, or Russia
Western Europe: Since Britain were guarantors of Belgium’s neutrality, their invasion by Germany in WWI during the Schlieffen plan was cited as Britain’s reason for entering the European war. In WWII, the Netherlands were invaded by Germany in order to draw Allied forces further north into Belgium, allowing the main German force to slip past them through the Ardennes, further to the south. I seem to recall reading that Portugal was also a hub for intelligence and espionage during the war.
TGW status: Minor Neutrals, can be influenced by UK, Germany, or France
Spain: I seem to recall in the years between German unification and WWI, that there were overtures made, trying to arrange a royal marriage between the German and Spanish monarchies, thus creating a threat to France from both sides. Spain remained neutral through the First World War, as well as WWII.
TGW status: Major Neutral, supporting either UK or Austria