You are generally on military land for quite some distance before you get to the actual gate -- at least that's been the case for every facility I've visited.
Usually there's a large sign "Foo Air Force Base" (or whatever) by the civilian road, then you turn off and drive on a military road for half a mile or more (sometimes much more), then you come to the gate with the guards.
But you've still been on military land, and subject to military rules and laws, from the instant you enter their property.
Edit: just checked. The guardhouses for two of the gates at a local facility are respectively about 800/1,000 feet away from the public road, and about 600/800 feet inside the perimeter fence (the one with signs reading "patrolled by dogs", "deadly force authorized", etc.).
Usually there's a large sign "Foo Air Force Base" (or whatever) by the civilian road, then you turn off and drive on a military road for half a mile or more (sometimes much more), then you come to the gate with the guards.
But you've still been on military land, and subject to military rules and laws, from the instant you enter their property.
Edit: just checked. The guardhouses for two of the gates at a local facility are respectively about 800/1,000 feet away from the public road, and about 600/800 feet inside the perimeter fence (the one with signs reading "patrolled by dogs", "deadly force authorized", etc.).