Defense

There are many factors in this game that give the defender an advantage; trenches, machine guns, railroads, militia, etc. It is not always appropriate to be attacking, and one of your primary goals should be to keep your units alive. Ultimately the game comes down to resources, and a wise player defends if they cannot attack efficiently. This generally involves building a lot of trenches along your fronts, and also along the coast where landings could occur.

Offense

It is important to go into every game with a plan of attack. You should attack wherever the enemy is weakest, and where you can capture the most settlements. When you capture one, you you only gradually gain income, as its population recovers. Your opponent however, loses their income immediately. The bigger impact of taking a settlement therefore, is not the the income it gives you, but rather the income it denies your opponent. For this reason it is generally better to attack enemies than neutral forces.

In addition to capturing settlements, it is also useful to capture some areas for tactical reasons. For instance, Austria can capture northern Italy, preventing French and Italian forces from linking up with each other. On the other hand, Austria can defeat Serbia quickly, thus freeing up Austrian and Bulgarian units from having to guard against Serbian attack.

Big Picture Strategy

You need to be aware of all your fronts. For Italy, there is only one with Austria. For Russia there are 4, spread over a huge area. Try to defend them all, but you should generally focus your attacks on only one. By doing this you are more likely to break your opponents defenses completely, taking large amounts of land. On the other hand, you should be aware of the global situation, and ready to change your strategy if an ally desperately needs help.

It is important to note that sometimes you shouldn't attack at all. Lets say you are playing as Germany, and all the Entente players have chosen their targets. France and Russia attack you, Italy attacks Austria, Serbia attacks Bulgaria, and Britain attacks the Ottomans. You will likely have a hard time just defending, as France and Russia combined have a much higher income than you. Your allies however are all up against smaller opponents. Your goal is to delay the enemy advance, and punish them if they over extend.

How to Attack

When attacking, there are two general strategies, the infantry charge, and the slow advance. The charge uses a mobile army to drive quickly through enemy defenses, and capture large amounts of territory. The slow advance uses long range units to destroy enemy defenses from afar, advancing slowly but safely.

Infantry Charge

You can rush with mass riflemen, using their fast speed and decent damage to punch through enemy defenses, taking as much land as possible. It's useful to bring machine guns and field guns also, to protect against enemy counter attacks. It is important to recognize when the enemy is too well defended for you to continue your attack, so you can stop and build trenches without excessive losses.

Once you have researched tanks, you can use them to lead the charge. Tanks have too little damage to be useful on their own, but they are excellent for absorbing field gun and machine gun fire, thus protecting your advancing infantry from harm.

Defending the Infantry Charge

When defending a charge, it is important you have lots of trenches near the front. The goal is to kill as many enemy units, as fast as possible, so having machine guns and field guns is very helpful. Enemy infantry can get in your empty trenches, so if you have some empty ones, salvage them. When the enemy attacks your line, spawn militia from your nearby cities, and counter-attack. Against small numbers of riflmen, machine guns are the most effective defense. Against large numbers however, field guns can make full use of their splash damage, and are the best choice.

The Slow Advance

Sometimes your opponent has hardened their defenses, such that a charge would be suicide. In this case, you can use mortars, howitzers and railways guns to destroy enemy defenses from afar. As these units advance, you must also advance your trench line to keep them them protected.

Defending the Slow Advance

When defending the slow advance, its best to have loosely spaced trenches, with lots of riflemen. Since the enemy is destroying your trenches from afar, there is no point investing in machine guns. The goal is to slow down the enemy advance as much as possible, and use your own artillery to fight back. If your opponent's artillery or mortars are ever left vulnerable, be ready to charge and destroy them with infantry. As the enemy destroys your trenches, save the men inside and bring them to a hospital. It's free to build more trenches, but it's expensive to keep replacing units.

Navy

For the more coastal nations (Britain, Italy, and the Ottomans), navy can be an important element of the game. There are several benefits to dominating the sea; you can capture coastal settlements, prevent enemy landings, escort your own landings, and bombard enemy coastal units. Battleships can contribute to all of these operations, and are the core naval unit. The downside is that they cannot detect submarines or mines, making them very vulnerable on their own. Destroyers are less efficient in a fight, but they are faster and can detect submarines and mines.

If your goal is to build up a navy that can attack enemy coastlines, you should spend 50/50 on battleships and destroyers. If your goal is to defeat an enemy navy however, it is better to focus on laying mines and patrolling with submarine packs.

Air Units

Air units cannot attack ground units, but are useful because they can see very far, and can see over forests and mountains. This can help you notice an enemy attack earlier, scout enemy defenses before you charge, or give vision to your artillery. Even if there are no obstacles, artillery can shoot farther than they can see. Keeping observation balloons near your artillery ensures they always make full use of their range.

Airplanes cannot see quite as far as balloons, but they can shoot enemy air units. Enemy machine guns, fortresses, and riflemen can shoot air units, so any flights over enemy lines must be brief. By getting several airplanes together (3-5 is recommended), you can often destroy a balloon and leave before losing a plane. Until the enemy replaces the balloon, you can take advantage of their lack of vision.