The chopper sequence is comprised of four main elements: a matte painting for the background, the chopper, the hero subject, and the foreground soldiers.
The initial setup involved extracting the hero subject from the live-action plate that was captured on location. Any areas of the subject that were not covered by the green screen were rotoscoped.
After keying the subject, 3D camera data was extracted from the footage. This allowed for 3D elements, such as the helicopter, to have real parallax as the camera moved through space. 
The chopper was then lit to match the daytime lighting of the live-action footage. Additional textures and decals were placed on the chopper in Nuke by projecting images onto 3D cards, and then moving those cards into the correct position using point cloud data.
The ground from the original footage was then expanded and cleaned up to eliminate any stray shadows, and then re-projected onto a 3D ground plane so that it would integrate into the scene seamlessly.
Soldiers were extracted from separate footage and composited into the scene.
The final phase included color correcting all elements to match, tweaking black levels, and compositing atmospheric elements for the final effect.