Anime is less a specialised form of art than it is a general collection of styles originating in Japan. There are no rules set in stone when it comes to art, and especially when it comes to manga and anime drawings. A few simple embellishments and flourishes of style are enough to draw an anime US Army soldier.
Examine your reference material, which can be anything from a photograph of a U.S. Army solider to an action figure or even someone dressed up as a solider. Draw a basic stick figure version of the soldier on your paper to get the right shape on the page. Picture the basic shapes that form the solider and sketch these on your paper over the stick figure.
Draw a more accurate set of lines within the basic shapes you have composed, to depict the actual shape of the head, torso, feet and so forth of the solider. The purpose here is to produce an outline of each body part and also equipment the solider is carrying, such as a backpack or weapon.
Draw in facial details, including eyes, nose, mouth and eyebrows. Add detail to the uniform and boots, including any pockets and camouflage marks on the soldier's clothes. If the solider is carrying a gun, construct it from basic shapes and then add detail to it in the same way as you did for the solider.
Go over your pencil markings in ink with your pen, so you have all the important lines inked. Allow the ink time to try. Erase all the pencil markings from the page, leaving only the inked copy of the soldier. If you forgot to ink any lines before erasing the pencil sketch, ink those lines in now.
Although it is by no means a hard and fast rule, anime and manga often use exaggerated body shapes to depict strong people, such as soldiers. Drawing a more powerful-looking person by exaggerating shoulder width and muscle tone will help give your drawing an anime feel. Alternatively, depicting a more slender figure will give the impression of a less combat-oriented soldier, such as a battlefield technician.
Try to avoid the temptation to get bogged down in minute details. Anime and manga drawings often forsake extraneous details in favour of a more streamlined, dynamic drawing. Too much detail will also detract from the drawing by making it hard for the eye to focus on the overall shape of the soldier because the drawing becomes too "busy".