AFAIK it's the machine gun that really led to entrenched positions because the death toll of charging or advancing on such positions was huge. Early in the war,

AFAIK it's the machine gun that really led to entrenched positions because the death toll of charging or advancing on such positions was huge. Early in the war, generals (who are known for fighting the last war) kept trying. This is one of the reasons the casualties were so high (sometimes >100K in a single day). But the real story of WWI (IMHO) was artillery. Here's a story I heard: at the start of WWI, Russia produced 35,000 artillery shells per month and was using 25,000 a day. By 1917, there were battles on the Western Front where between both sides over a million artillery shells were fired in a matter of hours. As for tanks, this was clearly a huge development. It really nullified machine guns and small arms fire. But the writing was already on the wall for trench warfare. Artillery accuracy and tactics had improved (eg the creeping barrage became possible) and tactics in trench assault (on both sides) were improving.