Battleground (1949) --- About the soldiers of the 101st Airborne in the Battle of the Bulge. Oscars went to script and photography. The first realistic WWII war movie made. Starring Van Johnson, James Whitmore, Ricardo Montalban, George Murphy and John Hodiak. All solid actors. Made only four years after the actual battle. Not all the flag-waving and trite scenes of the John Wayne war movies (which, in my opnion, are lousy).
The Beast (1988) --- Russian tankers lost in Afghanistan, trying to get back to their base, while the Afghans stalk them. Some trite scenes, and they get out of impossible situations (shades of Alister McClean!) but interesting because you feel sympathy for the Russians. Starring Steven Bauer and George Dzunda. Good actors.
Go Tell the Spartans (1978) --- Burt Lancaster as a commander of a small group of Special Forces during the very early years of Vietnam. All the while, Lancaster has grave doubts about what the U.S. is doing there. "Ah, you should have been at Anzio, now there was a war!" he tells Craig Wasson, a young soldier. While watching it, you just get this sense of doom because you know things will only get deeper and worse.
Story of Ernie Pyle (about 1953) --- Kinda corny in parts, but tries to tel the story of the famous war correspondent Ernie Pyle, in World War II. Pyle played by Burgess Meredith, a remarkable actor. This one may be out of print but no matter, find "Brave Men" or "Here is Your War" and other books and read his dispatches from the front. Easy to find in any good used book store, at $5 or so. He was killed on Okinawa months before the war ended. He's buried in the National Cemetery in Hawaii. Amazing writer. I wish I could find the single paragraph he wrote, describing the strafing of U.S. troops by a German Messershmitt. The economy of words, and how he was able to make you see and feel it, are remarkable. One of the finest examples of writing I've ever encountered --- and only one paragraph!
Hollywood needs to make a more modern movie on Ernie Pyle. He deserves it.
The Lost Batallion (2002?) --- With Rick Schroeder as the major in charge during a horrendous World War I battle. I think it was a New York outfit that was surrounded by the Germans and endured shelling, snipers and flamethrowers but didn't give in. A good view at the Hell that was World War I.
Based on a true story. They took very heavy casualties, then retreated at the end when ordered. A few weeks later, the war ended. The actual major later apparently killed himself by jumping off a cruise ship years later. Folks said he bore tremendous guilt for following his orders, and that so many good men died for no apparent reason.
And others that have already been mentioned: Cross of Iron, Das Boot, 84 Charlie Mopic (virtually unknown but needs to be seen), Patton, All Quiet on the Western Front (original, silent version but the remake isn't bad), Saving Private Ryan, Full Metal Jacket, Big Red One (the author and director served in the 1st Infantry Division in WWII and based some of it on personal experience), Band of Brothers, On the Beach (give ya the willies for a day or two), and The Dirty Dozen (Lee Marvin, the actor, was a Marine in the Pacific and had two or three beach landings under his belt. Not realistic, but vastly entertaining).
Some of the worst: Any John Wayne war movie, anything by Pauly Shore, The Battle of the Bulge (my father, a BoB veteran, watched that and was thoroughly disgusted with its inaccuracies and corny story lines. And at the end, a big battle in MUD! Not the snow and ice that covered Belgium during the battle! Ack!), Apocalypse Now (oh puhleeeez), To Hell and Back (corny and unrealistic, but typical of a war movie of that era. I very much admire Audie Murphy, though), any Rambo movie, The Thin Red Line (couldn't decide whether it was a war, love or philosophy movie), and The Dirty Dozen II (made by Lee Marvin so he could get some fast bucks and pay off his palimony).