Assuming it was technological development that drove new tactics, when and how were these tactics first learned and employed, such that by 1914 warfare was conducted along huge fronts defined by entrenched positions?
I'm interested in conflicts that might be seen as bridges between the tactics and uniforms of the Napoleonic wars and WW1.
Or perhaps WW1 was the first time 'conventional' warfare proved impossible/fruitless?
I'd be glad to know of any accessible books on the subject.
Although this is more /u/elos wheelhouse I can answer it decently.
First we have the development of the Machine Gun. The Maxim machine gun allowed for incredible volume of fire down range at an unprecedented speed. For comparison, most small arms were bolt action rifles that were stockpiled a couple years prior.
That's what would enable entrenchment and allow for a much more viable defense. The second and arguably more deadly weapon was the advent of much more powerful artillery. By the 1900s advances in metallurgy, rifling, shell dynamics (shape, etc) and production capacity allowed for artillery to rapidly outclass defensive capabilities and create much longer engagement ranges. By WWI artillery was a huge force multiplier in defense and offense because of the sheer area of denial artillery offered.
But that's the weaponry. The reality was, in 1914 in the early stages of the war, casualties were high, but trench warfare did not start until the race to the channel begun in a series of encirclement attempts by both the Allies and the Central powers.
Unfortunately my memory is failing me, so if an expert would step in I'd appreciate it.
Trench warfare developed during the first few months of 1914. When the war began it was fought over huge areas of open land in Belgium and Alsace Lorraine. In the former Germany smashed through Belgian forces and eventually besieged a series of forts that allowed them to turn south into France. From there they began driving into France and nearing Paris before being stopped at the battle of the Marne. The Germans found themselves unable to break through, so they fell back and dug in. When French counterattacks failed they followed suit. The same began occurring all along the front.
This was primarily due to new technology that no one was quite sure how to use yet. Neither side had developed the tactical coordination between artillery and infantry to flatten out barbed wire and smash trenches effectively. As the two sides kept trying to wrest territory from one another the coordination improved, but so did the defensive tactics and counter battery power of the Germans and allies, leaving neither side able to break the front and creating the stagnant trench lines we associate with WWI today.