So many great ones, I had to do 10...
1. Casablanca
2. Devils on the Doorstep
3. Kanal
4. The Cranes Are Flying
5. The Thin Red Line
6. Shoah
7. The Americanization of Emily
8. The Burmese Harp
9. Fires on the Plain
10. The Bridge on the River Kwai
Honorable mention: The Human Condition, A Matter of Life and Death, Mother Night, Lacombe Lucien, The Pianist, Night and Fog, The Way to the Stars, Army of Shadows, The Ascent, Das Boot, Downfall, Ivan's Childhood
Great, but I wouldn't necessarily call them "WW2 Movies": A Man Escaped, Underground, Black Rain