Why We Fight is a series of seven documentary films commissioned by the United States government during World War II whose purpose was to show American soldiers the reason for U.S. involvement in the war. Later on they were also shown to the general U.S. public to persuade them to support American involvement in the war.
Each of them is in the common domain having been produced by the US government, available online, and linked below the fold: [more inside]
Channel 4's 100 Greatest War Films
as voted for by their (generally more clued-up than average) viewership has plenty for you to disagree with, but much to recommend. Filmsite.org has a history of war films
(as does Berkeley
) for the completists among you. There are more war films from and about Vietnam
than you can shake a bayonet at (see also the 1999 NYT article, Apocalypse Then: Vietnam Marketing War Films
to learn a little about the Vietnamese government's 1960s and 70s archive of war film). The [British] national archives have archived film from pre-WWI to the Cold War