FWIW

How did they build that?

It looks like a tank training dirt field behind an Army camp. 

I would presume they drew lines in the sand and had all the personnel stand inside of that line, but with 18,000 people, the scale of that thing is absolutely huge to say the least.

I think they had the majority of enlisted personnel in their camouflage fatigues fill the area first and then had officers in their khaki dress uniforms fill in certain areas where you see the lighter color shades.

And then finally, they probably had a helicopter go up and take an ariel picture of it.

FWIW

I didn’t realize it was an aerial photo. It’s very clever.

Well, if you think about it, 18,000 people is really quite a lot.

Madison Square Garden can hold 18,000 people, but it is multi-tiered. Pope Francis holds mass for 18,000 people in Madison Square Garden – video | World news | The Guardian

Now spread them all out on a single floor and that's what you'll end up with. 

It's too large to take such a picture other than via an ariel view.

Also, look at the buildings in the background, all single story or two story buildings, not to mention all the forestry in the background. And as it's in black and white film, I would imagine the picture was probably taken between 1920 ~ 1950 timeframe.

FWIW

It’s a huge number and took a lot of planning.

Our military have always been adept to large scale operations with the objective of winning!

FWIW

Yes they have and if they weren’t doing anything and bored…