I think with every major conflict, WW1, WW2, Vietnam and the Gulf War(s) there's an initial period of seriousness that surrounds the conflict itself, where no serious film-maker would even conceive of lampooning it or poking fun at the brave men and women who laid down their lives during it. But it seems that lasts for maybe 15-20 years before a 'revisionist' period begins to creep in and we get films like Oh, What a Lovely War, or Kelly's Heroes, M.A.S.H, The Producers or Three Kings even.
I don't even know if Kelly's heroes could even be seen as a War film per se. Sure, it is set squarely toward the latter parts of WW2, but the war is completely incidental to the soldiers in this story and if anything it's an obstacle to what they're really interested in, which is the cache of gold bullion sitting in a bank that they've located behind enemy lines.
There are lots of reasons why this film is in this list; but the two main ones are first: although the soldiers in it are lazy and venal and for the most part, insubordinate, when motivated they prove themselves to additionally be, competent, resourceful ...and yes, brave, with all of the same attributes being being applied equally to their German adversaries. Indeed, when it proves impossible to get past Karl-Otto Alberty's Tiger tank Commander at the end of the film, they simply offer to cut him in on the deal to which he agrees at the drop of a hat.
Of course, the main irony of the film, is that in fighting their way behind enemy lines in order to get to the bank in the first place, Kelly's Heroes end up destroying a German supply depot, building a bridge across a strategically important river and liberating a French town, all unbeknownst to the bumbling General who is supposed to be leading them.
The other reason that this film is so good is that the cast is truly 'top-notch', from Kelly himself played by Clint Eastwood, the drug-fuelled Sherman tank Commander 'Oddball' played by a brilliant Donald Sutherland, the world-weary platoon Sergeant 'Big Joe' played by Telly Savalas all of the way through to Don Rickles' 'Crapgame' and Stuart Margolin's 'Little joe'.
War may be hell ...but why shouldn't you be able to make a few bucks along the way, eh?