The famous continuous line of trenches that ran from Switzerland to the English Channel came about entirely by accident as a result of the failure of the German forces to capture Paris in the opening weeks of the war. In the buildup to the war, Germany was always fearful of having to face a two-front war against the formal allies France and Russia. German leaders decided that their only chance of success was to throw almost all their forces against France and defeat her before the Russians could mobilize and advance into the eastern part of Germany.

France weathered this initial assault, and thanks to an unnecessary withdrawal of troops by Germany, she was able to halt the German advance outside of Paris at the First Battle of the Marne. At this point, the German armies withdrew a small distance and dug themselves in to favorable defensive ground. This initial defensive position of the Germans was essentially unassailable given the exhausted state of Allied troops, so the two sides began a series of attempted outflanking movements, trying to get around the opponent and cut off their supply lines. In this manner, the line separating the two sides extended itself in haphazard fashion southeast to the Swiss border and north to the English Channel. Once these limits were reached, the two sides dug in deeper along the entire stretch, and from that point until nearly the end of the war the line moved almost not at all.

So none of this trench line had been prepared in any way before the way. However, at certain places the line did pass through fortifications that had existed before the war, like the fortresses of Verdun. These were incorporated into the continuous trench line, so in a sense that part could be considered to have been built before the war.

Prior to WWII, France had constructed an extensive line of defensive fortifications (the Maginot Line) in anticipation of yet another attack from Germany, but nothing this extensive had existed before WWI.