The simplest form of GPS spoofing sends made up signals that misdirects the GPS receiver. Military equipment is protected against these kinds of attacks with authenticated signals. Carry-off attacks (satellite-lock takeover) start with broadcasting perfectly synced repeat of the original signal. This kind of attack can be used to make munitions and missiles to lose their signal lock in critical time.
It's also possible that there are multiple weaknesses and bugs in the military receivers that can be exploited.
Iranians were somehow able to trick RQ-170 drone to land on Iran.
When Iranians captured U.S. Navy patrol boat it was because there was mysterious navigation error.