I understand it is usually believed (see Germanic peoples (Wikipedia)) that up till roughly 500 BC the common ancestor of the Germanic languages (which today include English, Dutch, German, Danish, Swedish and Norwegian plus smaller languages) was probably spoken manly in what are now Denmark and Southern Sweden, but that thereafter they spread out and by around 400 AD dominated modern Germany, Holland, Flanders, Austria, Switzerland and also areas since lost to Slavic speakers in modern day Eastern Europe.
Why did this expansion take place? Was it mainly into previously Celtic speaking areas? If so, what enabled the Germans to defeat and/or assimilate the (Celtic?) speaking tribes previously in those areas during this period, when they were presumably not strong enough to do so earlier?
I know written sources are thin indeed from northern Europe in this period and more extensive Greek and Roman writings will be mainly based on second or third hand information, so it may not be easy to explain, but is there any evidence?
The expansion of Germanic speaking peoples into the crumbling former Western Roman Empire after c400 AD is better documented and while of course important (it explains among other things why you are reading this question in a Germanic language that originated in Britain after 400 AD) but is a different topic and not part of this question.