Question

How did the ancient Europe decline ? 

Answer 1

the ancient Europe decline mostly the ancient Roman Empire fell because it was unable to logistically maintain its military supremacy in Europe. This was despite the de facto control of the government by military leaders in place of the Emperor.

Answer 2

The first European settlers are Neanderthals.
the are an extinct species of human in the genus HOMO,possibly a subspecies of homo sapiens.there DNA just differ from modern humans is o.12%/
they extinct due to the climate change at the brake of snow age there body is well suited for survival in a cold climate ,Due to this climate change new plants and animal would be replaced by different ones within the a lifetime. they d't have new weapon to hunt where the grass land is replaced by forest.The home sapiens r more physical adaptive than neanderthals so they c't match with the advancement of home sapiens.