Erm, probably, but not in the way you think.
Like, everyone thinks that the act of stopping them using force, especially lethal will be difficult. Can't possibly shoot enough people, fast enough, to stop 1.6 million people. I'd say that if the US government really wanted to make an example and hose them all, it'd take maybe an hour or 2.
You only have to look at casualty statistics from WWI, and that was against people shooting back, using cover etc. A million people in open desert?
Easy. You wouldn't even have to go nuclear.
And I'd be a bit more creative than humvees fitted with Dillon miniguns.
I'd ask for 4x M270 MLRS, 12x flat racks of missiles (3 per launcher). A single 12 missile strike from one will reduce a 1000m x 1000m area of desert of all life, from Battle Tanks to Bacteria.
Or 3x AC-130 gunships.
Or a 6x B-52D Stratofortresses, big belly mods, and go all Arc Light on the place.
Nah, all that will happen is they'll break in, beat up some guards, break loads of things and discover not a single alien, anywhere. Because the US is many things, but dumb enough to keep proof of alien life in the exact place that everyone knows it is, it isn't.
And then once the crowd is done, and bored, and goes home, then the US will do its thing, and start arresting fools. But this is where things are going to get difficult. It doesn't have the facilities to detain anywhere near that amount of people for any length of time. Especially not in the middle of the Nevada desert.
You'd not even need a million people to get in. Anything over say, 10,000, and entry would be a forgone conclusion. You'd need the bigger numbers to get away afterwards though.