Optical quality glass is made all over the Pacific Rim and in Europe. The higher quality glass is made by Okuma in Japan and various sources in Europe. So an "ALL American" scope is non existent. NightForce, USO, and high end Leupolds use Okuma glass.
There is a very definite difference between the optical quality of European glass, and glass from the Pacific Rim. The deal breaker is the purity of the raw materials used to make the glass. The European source is of much better quality.
You may not notice much difference at close and intermediate ranges between top quality Japanese glass and European glass, but at extended ranges, low light conditions, or extremely hot climates, you will most definitely see a major difference. The clarity, resolution, and lack of distortion have to be seen to be believed.
I have a NightForce 12-42x50mm Target which has the very best quality glass Okuma offers on the counter and I let visitors to the shop take it outside and scan the mountains surrounding the valley we are located in. The distances that they are looking at are anywhere from just over 1,000 yards to a couple of miles. The next step is to hand them an IOR and do the same drill. Funny thing is that no one has come back inside and not been impressed by the difference.
I've said it for years now, and probably sound like a broken record, but European manufactured optical glass is the best that can be gotten at this point in time. If you can't see it, you can't shoot it.
I shoot the IOR scopes at night! Full dark! All I need is a little residual light (moon and starlight). I can't shoot the NighForce beyond full dusk because I can't see the target. Optical quality is what it is all about to my way of thinking.
Now let's discuss the mechanical aspects of a scope. USO, NightForce NXS series, Zeiss Hensoldt, IOR, and Leupold Tacticals are all hell for stout and will take a beating. They will survive rough field use that will kill most of the scopes that most of you are purchasing, and will stand up to the blast produced by Magnum and Super Magnum cartridges that have muzzle brakes (Muzzle blast is the big scope killer believe it or not.) A scope should also have REPEATABLE (very critical!) tracking. If it doesn't, it is useless to a shooter. There are very few scopes manufactured that have (again) REPEATABLE tracking. If you can't box the target with the scope and get repeatable (that word again) results, time after time, to me, the scope is worthless. You won't be able to dial in come up and windage values, and then return to dead nuts zero, each and every time.