Older movies are always important, but especially so for this genre. These are the movies people saw immediately following the war. It is important to see depictions of the enemy as well as how the boots on the ground perspective was told in the various stages and theatres of WW2. Later movies will be written and star actors that were actually in the war.
I watched war movies in December. I sorted my watch list from then and then edited out all the WW1, Korean, and Vietnam films and this is the list I came up with. I also took out the vast majority of the blatantly obvious choices that will be suggested over and over by younger viewers. I was left with the following. I put a notation next to the ones that I thought were especially interesting or thought they deserve extra attention. These movies cover the war from the ground, sea, and air. Hell, these can be further divided into subgenres for frogmen, submarine warfare, and POW movies.
Battle of the Coral Sea (1959)
The Frogmen (1951)
In Harm's Way (1965)
Is Paris Burning? (1966) -This about the underground soldiers of Paris who aided allies and performed gorilla attacks. (Intermission at 2hr mark!)
King Rat (1965)
The Last Blitzkrieg (1959) - Told from the side of Grmans who perform undercover ops vs the Allies. A rare view of WW2 and weird to see the main character killing allied soldiers left and right.
Merrill's Marauders (1962)
The Negro Soldier (1944) - US War Department's documentary focusing on the contribution of blacks during WW2. Super interesting stuff here.
None But The Brave (1965) - My favorite WW2 movie of this era. American & Japanese soldiers are stranded on an island and have to work together. The ending was fucking crazy. The movie treated the Japanese with dignity and respect.
Operation Bullshine (1959) - Lighthearted Comedy that I enjoyed. May have had a few song & dance routines lol.
Play Dirty (1969)
Raid on Rommel (1971)
Retreat, Hell! (1952)
Sailor of the King (1953)
The Seas Shall Not Have Them (1954)
Sink The Bismark (1960)
Storming Juno (2010) - Canadians on D-Day
The Bridge at Remagen (1969)
The Wackiest Ship in the Navy (1960)
They Were Expendable (1945)
To Have and Have Not (1944)
Torpedo Run (1958)
Where Eagles Dare (1968)
Why We Fight: The Battle For Russia (1943) - US Propaganda film shown to soldiers. Does not pull punches. Terrible, real footage.
Yesterday's Enemy (1959) -What would you do in this situation? type of movie. Makes you question your morals during war time.
A Bridge Too Far (1977)
Ambush Bay (1966)
Battle of Britain (1969)
Battleground (1949)
Beachhead (1954)
Big Red One (1980)
Bombardier (1943)
Bon Voyage (1944)
Fat Man & Little Boy (1989) - Dramatization about the process behind the creation of the atomic bomb.
Fighting Seabees (1944)
Great Raid (2005)
Hacksaw Ridge (2016)
Kelly's Heroes (1970)
The Cut-Throats (1969) Pure schlock with tons of explicit nudity - part of the sexploitation craze of the time and worth seeing for a peek into that genre alone
Guns of the Navarone (1961)
12 O'clock High (1949)
A Midnight Clear (1982)
Edit: Another interesting perspective into this era is the movies that were made leading up to and including war time. These were the movies that soldiers would frequently be shown as a form of rest and relaxation. It's odd to see movies of that time and remember that a soldier as likely watching it the day before heading out to D-Day or a few days after returning from horrible front line battles.