I've been reading quite a bit about the first world war, and it seems that every country involved has, in one way or another, a "Field Marshal" rank. I see less of this during the second world war, but it seems to have been kept in the Wehrmacht.
So what happened? Is there a reason not many western countries keep the rank?
The rank does still exist. Looking at the British Army specifically, figures such as Sir John Dill, Archibald Wavell, Alan Brooke, Bernard Montgomery, Harold Alexander, Claude Auchinleck and William Slim were Field Marshals during the Second World War--so the rank was held by most of the big figures. The reason it's descended into obscurity since then is that there hasn't been a large enough war since 1945. The Second World War involved armies and army groups--very few wars since have involved more than divisional-sized forces. There are still Field Marshals but they are either been Chiefs of the General Staff, who are customarily promoted to that rank, or honorary appointments, so we don't hear as much about them as we did about the men orchestrating the huge clashes of the World Wars. The full list of British Field Marshals can be found [here](http://en.wikipedia.org/wiki/Field_marshal_(United_Kingdom)#List_of_Field_Marshals).
In the UK, field marshals only exist in war, and not in every war.