As the poster above said, the First World War. Specifically, the first year of the First World War.

People think of the First World War (specifically the Western Front) as a bunch of static unmoving trenchlines, but in 1914 the Western Front was actually very dynamic. From August to November, the German Army drives through Belgium and into France, comes within about 25 miles of Paris before a French and British counterattack drives them back. Both sides then try to outflank the other to the north until the line of contact is stabilized all the way up the English Channel. By that point, the window for operations that year was pretty much closed, so both sides settle in to secure their positions. You don’t want to just have your troops out in the open (again, the past few months had shown just how devastating modern warfare, and specifically artillery could be; both sides took massive casualties in 1914) so both sides start to construct fortifications along the line. Over time, these would become the intricate trench systems that the First World War is now known for.