Heavy Metal is born in the UK, the first time as a passing notion in the '70s and the second time, for real, with the New Wave of British Heavy Metal. Great music comes from the UK is the cliché. The US looks with reverence to the old country and also jealousy. "I can do that as well" they say, "and I can do it better!". There you have your Jag Panzer, with their three Judas Priest album's worth of riffs in their one song. Succeeding with excess.
The United States are a country constantly searching for history. One needs watch only a couple of the newer Scorsese films like "Gangs of New York" or "The Aviator" to see the great pains their psyche goes through to evangelize and invent upon some would say paltry two hundred years of recent activity. A country of immigrants trying to do the world one better: let's invent the perfect nation.
This is a weakness and a strength. Shoulders unburdened with the weight of an Aristotle or a Nietzsche, when the US get in on some cultural action, they do it with such earnestness and desire to augment ("put on steroids" is the ugly cliché I'm trying to avoid) that the mutant results are equally grotesque and fascinating. Yet, for all their enthusiasm, they usually move on to the next thing in increasingly brief allotments of time. Five years in the maximum.
Such was their involvement with metal music, between 1984 and 1990 or so. They took the basic formula of Heavy Metal and made it faster (speed metal), made it punkier (thrash metal) made it more shocking and weird (death metal) and they even tried to make it modernist (progressive metal). And then they were bored and done with it, they moved on to reinventing and augmenting different musics. Only very recently have they returned to savage the corpse of past inspirations again, I guess we must be running out of 'new' things to make 'newer'.
Here's where it becomes complicated, however. Europe isn't just the United Kingdom. Other countries around these parts that were in the sphere of cultural influence of America, due to the language barrier and other reasons did not notice the incongruity between NWOBHM and US metal, they took everything prima facie, a real history and an invented one both together, the grand Heavy Metal tree with all its various co-habitual branches. This was a misunderstanding, for as far as the US type of cultural thinking goes, once you augment the music you started with, once you take Heavy Metal and you create out of it 'Power Metal', then Power Metal has killed Heavy Metal. This is the anxiety of a country with little history: how to carve out a niche for oneself, how to ascertain one's continued existence. Roots must be invented, exploited, discarded, start again. "Thrash metal" wasn't meant to live side to side with old world metal, it was meant to replace it.
Europeans, due to naivety and perhaps lack-of-naivety as well, do not think in this way. They were impressed and inspired by the US boom of metal sub-genres and they took them and expanded on them infinitely, they found a place for invented history in real history, and that's how the story goes. Where the US is jealous of the artistry that comes with the management of the weight of history, the old world vampires are jealous of the spontinaety and vitality of the US.