The US military uses metric measurements more than civilians, but it is not universal. Ground forces measure distances in "
klicks", slang for kilometers. Most military firearms are measured in metric units, beginning with the
M-14 which was introduced in 1957, although a few legacy exceptions exist, such as .50-
cal guns.
Aircraft Ordnance is normally measured in pounds. Heavy weapon caliber is measured in millimeters. Military vehicles are generally built to metric standards. An exception is the U.S. Navy, whose guns are measured in inches and whose undersea fleet measures distances in terms of "kiloyards" (equivalent to 914.4 m), depth as "feet", and velocity, in some cases, as "feet per second". The Navy and Air Force continue to measure distance in nautical miles and speed in knots . . . Furthermore in military aviation NATO countries use feet for flight heights, as they do in the civilian aviation.
A lot of the usage is guided by NATO STANAGS (standardization agreements) with the goal of having all members use the same standards for measurement to minimize misunderstanding. Another possible but reasonable explanation for the military electing to use metric would be to "improve" relations with our allies. Nothing turns off your friends more than you accidentally getting them killed because you're using two different rulers.
For instance, all military maps are laid out in kilometer grid squares (or the Military Grid Reference System). Everything from Land Navigation to Mortar and Artillery Call for Fire (CFF) procedures are based off of the standardized mapping system. Thus, no matter what, when you look at a grid square, you know that halfway across the square is 500 meters, a tenth of the way across is 100 meters. If the grid squares were in miles, now you have a situation where Lance Corporal Hooyah is trying to divide 1760 by 2, or 5, or 10, or 1/4.