Being my first time answering a question on this subreddit, I will provide some credentials first, and the adress the question. I have a bachelors in history, focusing on war history, in particular post-Napoleonic war and the relationship between politics and warfare. Later switching over to political science for my masters degree I feel competent in answering this two-part question which I consider touches both on historical and current affairs. The first question, «Why was trench warfare so common in WW1?» deserves a multiperspective answer. I will mainly focus on the strategic and the technological perspectives. I will return to these perspectives when addressing the second question «[…] why is [trench warfare] so uncommon now?». These perspectives are mainly drawn from Rolf Hobson work, in particular «Krig og strategisk tenkning i Europa 1500-1945» (2009) [War and strategic thinking in Europe 1500 – 1945. Own translation of title]. Im also drawing on Nils Arne Sørensens overview book on WW1 «Den store krigen» [The Great War. Own translation of title]. As it was by and large the Western Front that was dominated by trench warfare I will focus on that geographical are for this answer.

The strategic element is connected to the military leaderships view on how to win battles and wars. These views was, according to Hobson, largely based on theoreticians Jomini and Clausewitz, whom again had studied, lectured and developed their perspectives on how Napoleon Bonaparte fought battles and waged war. These perspectives deserve a long elaboration, but to keep it short: the one who attacks wins – both the battle and the war. Clausewitz view on Napoleons idea of seeking the enemy and imposing a crushing defeat, can be seen again in the Prussian General Staff and chief tacticians Moltke and Schlieffen.

Keep in mind the long relatively peaceful period between Napoleons defeat and WW1. The only major war in Western Europe was 1870-1871 where Prussia defeated France in a short and intense war. This war, again referring to Hobson, showed the European countries that the strategic offensive, combined with the tactical defensive was the strongest form of warfare. However, Hobson argues it was the swift mobilization, strong organizational skills and use of railroads that gave Prussia the leverage to win despite the French having the tactical defensive advantage. The tactical defensive was severely strengthened by technological developments such as rifling, Minié bullets, and breechloading rifles.

So, at the beginning of the war in 1914 we have a situation were the technological and doctrinal developments gave the defender a huge advantage, but the strategies employed by the high command was offensive (such as the German Schlieffenplan and the French Plan XVII). Sørensen adds technological advances such as repeating rifles, artillery and machine guns to the arguments for the strong defensive advantage, as well as adding railroads and trucks as an argument for how to maintain the trench warfare. The extensive use of wire can also be added to the defensive advantages.

Returning to the strategic perspective. The German Schlieffenplan was offensive and was supposed to go fast. The core concept was to defeat the French, just like they did in 1871 and then turn east to face the Russians. The plan failed – both because it was far too ambitious (supposed to take the fortress at Liege in two days – it took 11 days) and also the massive turn around of the French army led by Joffre (ref. Sørensen). When neither the French nor the Germans was able to gain the strategic offensive during the so-called Race for the Atlantic, they dug in. Due to the technological and infrastructural advantages (railroads etc) of the defender it was close to impossible to move the front lines. The front lines barely moved until the developments of the tank (which provided cover for advancing troops), the doctrinal development of barrage artillery (essentially timing the artillery to shoot just ahead of advancing infantry), and the arrival of fresh American troops. All these factors coalesced with the blockade of Germany by the British Fleet, which starved and eventually turned the Home Front against the German military High Command.

Now turning to the second part: why is trench warfare so uncommon now? Keep in mind the technological and doctrinal developments since WW1. First of all, airplanes, helicopters and for a few superpowers: aircraft carriers. These are capable of bringing a lot of concentrated firepower to the enemy, instead of waiting for the enemy. The same can be said for the use of tanks and APCs – they are all offensive and armoured. On the doctrinal side we know quite a bit from the American perspective: Search and Destroy from the Vietnam War was offensive, and the Shock and Awe-doctrine employed during the First Gulf War demonstrated how modern offensive warfare could be done (many more factors, but these are some examples). And we should not forget the Blitzkrieg-tactics of Nazi-Germany, and the various adaptations employed by the Allies during WW2.

Essentially we see that the technological developments to make WW1 a dynamic war, also gave the aggressor the tactical advantage – for example being able to surround and destroy hundreds of thousands of Soviet soldiers during Operation Barbarossa. Post-WW2 developments of the assault rifles, M16 and AK47 probably the best known, also emphasized and gave the offensive. Further development and use of radio improved communication.

What we see in Ukraine is that the technological development have changed the face of war again. Russian tanks are being destroyed by advanced handheld anti-tank-weapons, therefore removing the offensive use of tanks, maybe altogether. However, we see the extensive Russian use of rockets as an example of defensive firepower, while the Ukrainians have been seen using ATVs for reconnaissance-missions and hit-and-run-attacks. By digging in, the Russians can deny the Ukrainians their mobility advantage, and keeping the advantage of more rockets and missiles. So keeping to basic human logic – if the enemy sends missiles towards you, and you are in a favorable position (Bakhmut) – what would you do? Probably dig in. Just as the Germans and the French did in 1914, the Ukrainians and Russians do in 2022.