Inspired by this question: https://www.reddit.com/r/AskHistorians/comments/6x63fj/in_wwi_trench_warfare_how_was_it_decided_who_had/?utm_content=title&utm_medium=hot&utm_source=reddit&utm_name=frontpage
The current top answer describes how an attack would have worked, and in doing so, implies what would happen if an attack was successful. But, a huge part of the narrative of the Western Front of WW1 is that the trenches were effectively unmoving and that the war was a stalemate for the majority of its duration.
Could someone go more in depth as to the reality of changing battle lines on the western front? My understanding of WW1 trenches was that a successful attack rarely happened in general, and if it did, it was quickly driven back. Is this accurate?
It varied quite a bit. Some areas remained largely static for the entire war (such as the Yser Front in the far north-west of Belgium, held by the Belgian army). But in other areas the front line could be extremely dynamic as both sides fought over important strong-points and geographical features to incorporate into their lines, expending huge effort to take hundreds or even just dozens of yards of useful ground; in some places battlefields became an absolute labyrinth of old trench systems, saps, dugouts and earthworks as each side sought to take small sections and then absorb them into their line, either as a means to dominate the local sector or as a step towards a larger breakthrough:
When moving about in the trenches you turn a corner every few yards, which makes it seem like walking in a maze. It is impossible to keep your sense of direction and infinitely tiring to proceed at all. When the trenches have been fought over the confusion becomes all the greater. Instead of neat, parallel trench lines, you make the best use of existing trenches which might run in any direction other than the one you would prefer, until an old battlefield, like that of the Somme, became a labyrinth of trenches without any plan.
Charles Edmunds (Carrington) Subaltern’s War (London, 1929) pp217-218
The term 'bite and hold' was sometimes applied to this strategy, and I wrote a little about how these (relatively) small actions took place in an answer to this thread.
One of the hardest things to comprehend about actions like these is how men achieved the seemingly impossible - constructing and linking up new trench systems in close proximity to the enemy - and the answer is really 'mostly at night and with appalling numbers of casualties'. Field Entrenchments: Spadework for Riflemen by John Solano (first published in 1914, with updates throughout the war) is a superb resource on this element of the war and can be viewed in its entirety on-line thanks to archive.org; it's well worth a look.