U.S. Military protects their brand

U.S. Military branches have trademarks and licensed products. And just as companies must protect their trademarks, the U.S. Military must also protect theirs. When private companies use military images in product promotion, things begin to get hazy, see - Military, Inc.: How Companies Use SEALs and Soldiers to Sell. Just how far can you go using military images without infringing on U.S. Military trademarks?