Britain
(Entente)
Starting income: 420/month
Britain is in a unique position, with two starting areas, the British Isles and Egypt. Britain has more coastlines than any other nation, and also starts with the largest starting navy. Your main concern as Britain is fighting the Ottomans, but helping France fight Germany is a viable option as well. The British Isles are not often attacked directly, but when it happens it can be devestating, and should be guarded against.
Special
Periodically recieves a transport full of riflemen from Canada. Transports arrive from the west, drop off their cargo at the nearest British city, then return to the west. Empty transports that return to the west are refunded. An exception to this is that the first two transports do not return automatically.
France
(Entente)
Starting income: 600/month
France is a major nation that only has one front, Germany. As a result, your strategy as France really depends on what Germany does. If Germany is focusing on you, play defensive. If Germany is focusing on Russia (or anywhere else), play more agressive.
Russia
(Entente)
Starting income: 720/month
Russia is the largest of the Entente nations, and in the unique position of having borders with all four members of the Central Powers. Russia is also the largest nation, with large distances between its cities. While Russia has railroads to help traverse the large distances, invading enemies do not. This makes Russia especially hard to invade, as it can easily fall back to establish new lines when overrun.
Special
In March 1917 (about 40 minutes into the game), the Russian Revolution occurs, and lasts until the end of 1918. During this time, neutral armies of riflemen and militia spawn randomly, and try to take over the country. These armies can appear anywhere in Russia, and attack anything they see. They even appear in areas that are no longer controlled by Russia.
Italy
(Entente)
Starting income: 285/month
Italy is similar to France, in that it has a single front with a larger opponent. As such, your strategy is dependant on what Austria does. The main difference is that Italy is also very coastal, which can pose a problem if the Ottomans come to dominate the sea.
Serbia
(Entente)
Starting income: 150/month
Serbia is the weakest nation in the game, and is in the unfortunate position of being stuck between Austria and Bulgaria. If only Bulgaria attacks, Serbia has a chance of holding on, or even winning. If Austria attacks however, Serbia is likely doomed, and should focus on delaying the enemy.
Special
If Serbia is defeated or surrenders, the player takes control of Portugal instead. Portugal is even smaller than Serbia, and is removed from the usual fronts. In order to get back into the fight, Portugal must either attack through neutral Spain and then fight in France/Italy, or bypass Spain with transports.
Germany
(Central Powers)
Starting income: 750/month
Germany is the most powerful nation in the game, with the most income, and also the largest starting army. Unfortunately, it is also sandwiched between two large opponents, France and Russia. it's large size, plus it's precarious position, make Germany the most important nation in the game.
Austria-Hungary
(Central Powers)
Starting income: 600/month
Austria is a major nation that is surrounded by three enemies, Russia, Italy, and Serbia. Russia is the most dangerous, but is often occupied by your allies. Italy is small, but only has one front to focus on, and is the main threat. Serbia is not very dangerous, as it is very small, and usually occupied with Bulgaria from the start.
The Ottoman Empire
(Central Powers)
Starting income: 600/month
The Ottoman Empire is a major nation with two small fronts, one with Russia, and one with british Egypt. The Ottomans are very coastal, and war with the British often involves significant naval action. On land, narrow fronts mean that it's relatively easy to defend the infantry rush, and artillery become especially useful.
Special
Capitol city is Istanbul, which is located beside a bridge crossing the Bosphorus strait. Whoever controls Istanbul also gets control of the bridge, and it can be used by anyone on their team. To use the bridge, select some units and right click on it.
Bulgaria
(Central Powers)
Starting income: 225/month
Bulgaria is the smallest nation on the Central Powers, and has borders with both Serbia and Russia. Serbia is the most natural opponent, and in this fight Bulgaria has the advantage. Technically, neutral Romania lies between Bulgaria and Russia, and this often discourages fighting between the two nations. However, Serbia is often defeated before too long, and once this happens Bulgaria can turn its attention on Russia.