Security is a weird issue. Not every facility is Fort Knox or NORAD. They don't all have massive walls. Area 51 has traditionally handled security in a very subtle way. There is no fence. They simply have a vague perimeter marked by orange posts every few hundred feet. They almost "invite" you in. Thing is, once you are past the perimeter, they do not have to answer any questions and can do what they want to you. You have many miles of desert where they can easily intercept you, by land or air. And that's not even the real base. The desert just leads to Groom Lake. The real secrets of Area 51 are located miles back into the mountains, at S4, and then underground. You just can't travel 30 or 40 miles through hostile territory without being intercepted. And once captured, you're already trespassing, already in violation. You're screwed. The security is formidable to say the least.
Granted, in 1957, Area 51 was a smaller facility. The S4 site may not have been built yet. I could easily image the Roswell wreckage sitting in a warehouse just as "Kingdom" depicted it. But even then, as Montana pointed out, if a rogue group broke the perimeter, the Russians could easily deny involvement. In fact, Spalko's team probably had that in their briefing. It's standard black op protocol: "If you're captured, we will deny any involvement or knowledge of your existence" (thank you, Rambo).
And yes, Lance, the secrets at Area 51 were well worth the lives of some soldiers, in the eyes of those in charge. Even were it not for the alien artifacts kept there, the development of secret planes (the U2, Blackbeard, etc.) were enough of a technological and financing secret that the folks in charge would have just written those guys off without a second thought.
You've got a good heart, Lance. You have idealism and like to think better of your government and fellow man. But I'm afraid you really underestimate just how ruthless people can be when keeping secrets. Not just the American military, but all military and government. It's human nature. The more they have to protect, the more ruthless they will be. I mean, damn, they're willing to kill civilians just for entering the 27-mile exclusion zone (the signs clearly posted there say, as is well known, "Use of deadly force authorized". If they'll kill their own civilians just for snooping, they'd sure as hell not care about soldiers whose job it is to die defending the country. And make no mistake about it, the powers-that-be consider keeping these secrets critical to "defending the country". As far as they're concerned, it's a war not of field battle, but of secrets, a battle the public doesn't need to see. The soldiers at Area 51 would just be casualties -- a few among countless others who die in black operations year in and year out.
Spalko's penetration of Area 51 would be, at most, explained to the families as a "training accident", while behind-the-scenes American agents would be yelling at Russian agents who'd be denying the whole thing. Then, behind THOSE scenes, the American equivalent of Spalko's team would be sneaking into some Russian facility.