The US military has always used inertial navigation systems, usually based on extremely precise laser interferometers. You can't spoof or jam inertial guidance short of locally altering the laws of physics. A few decades ago, GPS was used to apply corrections within the (classified) error bounds of the inertial navigation system, which could be significant; any GPS correction outside the error bound of the inertial navigation system was interpreted as GPS being compromised. As the decades have passed, inertial navigation systems have become progressively more precise to the point that GPS is adding a rapidly shrinking amount of extra precision.
In fact, the US military is starting to test a new type of ultra-precise interferometer that allows inertial navigation to exceed the precision of GPS. GPS correct INS will only continue to be used to the extent it is inexpensive and gets the job done.