As futile as the concept is, it does make for an interesting mental exercise...

Obviously a "million vehicle onslaught" would never work - they'd "see it coming" so easily and assuming lethal force was actually used, well forget about it. But I don't even think it'd come to that - they'd just "close" a few surrounding roads and that's the end of that...

On foot? Forget about that 1000x - that's seriously remote desert out there, "Area 51" itself probably wouldn't have to even lift a finger.

But let's say you're really serious about getting in - NOT that you're going to get inside any BUILDING there, but just to get inside and trespass. Just to see the place with your own eyes (a pointless and stupid goal...)

Interestingly, if you could somehow find a way to deal with little things like WATER you might actually have better odds (solo or a pair) of pulling off an incursion (but nothing more than exactly that) than some "state actor" actually trying to pull off an incursion and actual mission goal of some sort...

I'm thinking you'd have to be some kind of "survivalist" who really knows how to handle the desert to even begin to think of such an endeavor...

And even then I think all you'd accomplish is an incursion...