Its called “They shall not grow old” and it was created by Peter Jackson (Lord of the Rings, The Hobbit, etc.) and the British War Museum to mark the end of World War I.
World War I really was the turning point of the modern world. What Peter Jackson did was take the footage from the British Museum (the old, grainy black & white, silent movies) and digitize it/clean it up so it can be viewed today. He then colorized it and added sound effects/voices so that it is just as good as movies made today.
All the voices are from actual WWI veterans, recordings the BBC had from years ago.
Its a monumental task, and it makes a gripping movie. At the end, Peter Jackson spends about 25 minutes going through all the technology that was used to make it happen – and a great & funny story about how the closing song for the credits was made.
All the reviews are 5 stars on Rotten Tomatoes, and I can’t recommend it enough. It had a short run (one weekend) back in December, but it is back in the US for another short run. Go see it!