In most cases, the safest strategy is to maintain a defense around depots and factories until the enemy runs out of troops. Take, as a simple example, a depot whose gates are protected by a few infantry units and one bunker, with the depot containing one full-strength cavalry unit. During a movement turn, select a frontline infantry unit that is likely to be attacked in that turn – because it is exposed or because you saw on the computer’s screen that it has been selected as a target. Move it back to the depot, and put in its place the cavalry unit from the depot. As the orders of the turn are executed, your withdrawing infantry unit will come under fire but also shoot back, so that it will actually arrive in the depot with reduced strength but increased experience. During the subsequent attack turn, repair the infantry unit which you just pulled back. The replacement cavalry unit should not attack, because the point of this strategy is to stall for time. Instead, only the bunker should fire, but only at enemy units that are not right next to it, because otherwise they will shoot back during the barrage which needlessly shortens the bunker’s lifespan. During the next movement turn, move the cavalry unit back to the depot where it came from and replace it with the infantry unit that you had withdrawn and repaired in the previous turn: it will resume its original place with full group strength and increased experience. The more reserve units you have, the more frontline units you can replace like this. Your line will not have moved, your units have only become stronger, while the bunker keeps firing away. The more bunkers and artillery you have in front of the depot, the more devastating a protracted siege will be for the enemy. In principle, this routine can continue endlessly. In practice, you will need to break out and charge forward when the enemy’s medium and heavy artillery starts coming within range. Since the computer uses uncoordinated frontal assault tactics, his infantry and armor will typically race forward and be spent by the time his artillery catches up with it. Destroy it when it does, though: the safest place around medium and heavy artillery is right in front of it, because it cannot fire at adjacent hexes. When this is done the road is clear.
Even when there is no depot, factory or HQ around (all three of them can repair units), a defense against an enemy with superior numbers is still easier to conduct than an assault. In fact, in the real WWI the defender had the advantage as well because firepower was more advanced than mobility. The main rule in the game is to always keep other ground units between your artillery and the enemy, so that the guns can keep firing at full group strength without being attacked themselves.
If you do launch an assault, the key is usually to have air superiority and mobile artillery to quickly weaken or wipe out enemy strongholds. To gain air superiority, use AA emplacements, elite infantry and aircraft – in that order – against enemy aircraft, and engage enemy AA emplacements as priority targets. Armored trains and some ship types can shoot down airplanes as well. To employ mobile artillery, move light artillery and anti-tank guns onto supply trucks, wait one movement turn, and then assault: the trucks will carry the artillery close to the enemy’s lines and drop it off within the same movement turn so that they can start firing right away; remember medium and heavy artillery need one movement turn before they can start firing, so they are best used for lengthy engagements or sieges. If you have factories, depending on the situation the most effective units to produce are usually aircraft and light artillery.