Probably the best strategic WWI game out there, though that doesn't say much considering how few there are but it is still pretty good. The combat system for the Strategic Command series works well for this time period, and lends itself to some pretty deep tactical and strategic decision making.

On the strategic side, you have quite a few options available to shape the direction of the war. As the Central Powers right off the bat, you decide whether to declare war on Belgium and focus on the west or the east, or to focus on Serbia or Russia. Do you build up your commerce raiders and subs, or build the surface fleet up to break the blockade? If you raid, do you go after targets that will do the most damage, but also drag the US into the war or not? Do you send Lenin to Russia and risk his agitation affecting you later, or fight the war conventionally? Do you give Italy territory to keep them neutral, or risk war on the Alpine front? And lots more. Similarly, there are a plethora of options for the Entente.

Tactically, the game captures the evolution of modern war fairly well. Early on you have your basic army corps who can pack a punch in the open field but will also take one, but as soon as you face down entrenchments you watch the combat slow down and attacking becomes much riskier. To Break the Deadlock you have to invest in your artillery, airpower, infantry weapons, and tactics and command. All of these become critical as the war drags on and on. You need to carefully plan offensives around maintaining your lines of supply, especially in the more rural theaters of war.

There's a lot to say about the game, but really just diving in and playing it will help the most. If you are interested in the period at the strategic level, its worth picking up.