This might be of some interest to you,
BeinDk.
As far as my own thoughts go... I had just prepared a fairly long post, but it has been lost. Here's the main points, since I really don't have the time to type it all up again.
Similarities:
1. War in both
2. Rohan coming to Minas Tirith <=> D-Day Invasion
3. Sauron used a strategy similar to Germany's
blitzkreig(sp)
4. Both wars had two fronts
Dissimilarities:
1. Mordor/Isengard was doing sandwiching(Isengard-->Rohan/Minas Tirith<--Mordor); Germany was sandwiched(US/UK-->Germany<--Russia)
2. Overwhelming odds in LotR not seen in WWII
I was also thinking of comparing the Ring, being the bane of Sauron/Mordor, as being the equivalent of fuel and oil to Germany. The Axis' lack of fuel towards the end of war was one reason they lost.
Sorry I can't expand anymore on any of that. Hope it helps.