Operation Tumbler-Snapper was one of the most controversial series of atomic bomb tests conducted by the U.S. Military. In one of the tests, 1000 troops were placed in trenches just 7000 yards (3.9 miles) from ground zero. Oppenheimer apparently warned them about the deadly effects on the troops at that range, but the military proceeded regardless. Skip to 40 minutes in for this particular part of the test, with the actual detonation 3 minutes later in the video.

"Operation Tumbler-Snapper consisted of eight nuclear shots in two phases. The Tumbler phase was of primary concern to the Department of Defense, which called for airdropped nuclear weapons tests. The Snapper second phase was a set of experiments conducted by the Atomic Energy Commission and the Los Alamos Scientific Laboratory to help improve effects of nuclear weapons." - YouTube.

See YouTube description for much more information.

Also, be sure to check out this photo, which is an x-ray photo taken milliseconds after detonation of the "Snapper" phase of the tests. Note the faint metal tower below the fireball, which the bomb was placed at the top of.

I know the video is long, but the footage is shocking in places.