As already stated, the lack of a central government among the Celtic tribes, made the great expansion the reason for their decline.
The Celts were the first people to submit to the Roman Empire, so much so that at the end of the 2nd century BC. Cisalpine Gaul and Celtiberia were already conquered territories. Under the command of Julius Caesar, in the 1st century BC, Transalpine Gaul was taken and, in the same period, Galatia became a subordinate province of Rome. With Gaul already conquered, the legions advanced to the British Isles, where domination took place gradually and was completed at the end of the 1st century AD. Meanwhile, in this same period, the Celtic tribes in Central Europe fell under the rule of the Germanic peoples.
In theory, it was the end of La Tène culture and Celtic art, as conceived, ended up confined to the Island of Ireland, to flourish again in the early Middle Ages in an already Catholic Christian environment.
Aside from the Irish region, the tradition and languages of Celtic heritage still survived in the other regions inhabited by the Celts in the last years before domination, such as Cornwall, Manx Island and the Scottish Highlands (United Kingdom), Brittany (France), in Galicia (Spain) and Galatia (Turkey).