And even on the Western front, the thing did not start with trenches from the get go. During the first month or so, both sides wanted a maneuvered war, quick conquests, daring advances, and spent tens of thousands of young men in order to get that all.
So, while for a long-term non-trench-warfare war you do need stuff like lots of tanks, anti-barbed-wire devices, engineering vehicles (anti-trench, you see), air support and whatnot, you can also get by if you somehow manage to make the war end in the first one-three months. And preferably the first one.
A big POD, obviously. It also means that this freakish one-off experience will condition further developments of military art, and the shock of the trenches is only postponed to the next war - unless by then, you do get all the tech mentioned above.
Edit: I think it's worth adding that the possibility I mentioned in the last line, above, is pretty unlikely. If you had a 6-week war in 1914 with, presumably, the Germans overrunning in maneuvered battle the French, to then finish off the Russians... well, civilian trucks and postal aircraft get developed anyway. But tanks, Bangalore torpedoes, automatic weapons so light that you could fire them on the move, bombers, gases etc. were a solution to a problem, trench warfare, that has not happened. So they might exist as prototypes, but anything more is rather unlikely.