this most people know about the term 'arms race' but don't know how it is applied in reality a simplified version is > enemy has tank with 100mm armor than is impervious to your ap rounds which can only pierce 90mm > you develop new round which can pierce 110mm armor > enemy starts losing tanks, they investigate and see your rounds are getting through, conclude you have capability to pierce at least 115mm with a 5mm margin of error > enemy welds add-on plate to their tank to bring up the thickness to 120mm on and on for 60 years and now we have tanks with 1 meter thick armor it's not really about the technology. everything we have, like thermals, armor arrays, long rods, and so on, russia already has that stuff.  russia having the leo 2 means they can optimize their rounds to defeat its armor in the most efficient manner, as well as get exact measurements for how much armor the leo 2's apfsds can penetrate so they can weld some plates onto their tanks to make the armor just barely thick enough to make them impervious this sort of advantage is why every country that sells their own equipment always has 'export variants' with worse stats so that their enemies can't get their hands on them and make equipment to counter it efficiently.
United States