You can choose to play a full campaign as either the British or the Germans. Win each skirmish by reaching the opposite side of the battlefield with your troops, or alternately by reducing the enemy morale to the point of surrender. What makes Warfare 1917 interesting is the use of trenches as strategic choke points. Since your operating space is essentially a one-dimensional horizontal line, you must be conscious of each of your units' strength and firing range, both in and out of the dirt.
A system of experience points and a wide range of upgrades make the campaign worth replaying, and if you ever just want a quick battle, there is a customizable one-shot skirmish mode. Early versions of Warfare 1917 had some balance issues, especially once the virtually invincible tanks entered the fray, but version 1.2 has things hammered out pretty well. It's a real gem of a wargame that works on more levels than just pew pew pew KABOOM. Though certainly it provides that as well.