Because American weapons systems use so many electronics items, and so many of the components (especially the microprocessor type chips, and others of similar complexity) come from overseas (mainly East Asia), there has been growing fear that a hostile nation might slip sabotaged chips into U.S. weapons. Four years ago, this led to the American "Trust in Integrated Circuits" program, which has developed technology and techniques for verifying that chips going into military equipment, are not secretly modified to aid the enemy (by letting enemy hackers in, or failing under certain conditions). The Trust program is not finished yet, and it won't confer absolute protection, but will make it harder for enemy to slip in secret spy chips.
This, however, does not offer protection from another scary problem. It's the increasing sale of counterfeit computer equipment to military organizations. But it's not just about the selling of cheap knock-offs at premium prices, but the espionage potential of this sort of thing. The most vulnerable equipment is network cards. Crooks buy cheap, generic ones from China, then attach authentic looking stickers to make it appear like a top-of-the line item from an American supplier. The intel angle enters the equation when you realize that these cards could have special versions of chips that are hard wired to either allow foreign hackers access to U.S. networks, or "call home" to enable the foreign Cyber Warriors to know where their fake cards have ended up.
There have been plenty of counterfeit cards uncovered, but none yet with the spy chips installed. What's worrisome is that U.S. Cyber War units have apparently already created such spy chips, as these can be installed in legit equipment as well. All you have to do is get the target country to accept the gear and install it. Normally, and for obvious reasons, no one wants to talks about this sort of thing. But the growing number of counterfeit electronics being peddled to the U.S. military, and energetic FBI efforts to catch the counterfeiters, has made it more difficult to keep this form of espionage completely under wraps.