Until relatively late in antiquity various Celtic peoples predominated in parts of Anatolia, France, Spain and other areas in between. I have also heard that the republican Romans considered them fearsome warriors and may have even copied the Celtic javelin/sword/shield strategy to create the iconic legions.

How did this all change? Was it tied up with the 'migration period' or did it happen before? Why aren't there major, continental European nations that speak Celtic languages today? In a place like Turkey are there any vestiges of Celtic language/culture today?