Why were Germanic languages able to spread over much of northern Europe after 500BC? Did they mostly replace Celtic?

Question

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.

not necessarily "tied up with the "why" of the fall of the (western) Roman Empire" as this question relates to events before or contemporaneous with the Roman Empire c500 BC - 400 AD. My last paragraph was intended to explain that the expansion of Goths, Vandals, Franks, Angles, Saxons, Sueves etc. into what had been Roman provinces after c 400 AD is, while important, too large a topic in its own right to include in this one question

Answer 1

I'm seeing two different questions to address in here: What happened to the Celts, and Where did all these Germanics come from?

What happened to the Celts? They got culturally absorbed by the Romans.

The first thing that you should notice from the below two linguistic maps from 500 BC and AD is that the Green Celtic areas have been almost entirely absorbed by blue Italic. Some is also now purple (for Germanic) but all of that area was first taken over by the Romans, and most continues to speak Romance languages to this day. There was possibly as much voluntary absorption as there was genocide on Celts, but the modern result is the same. If you are looking for culprits, you need look no further than the Romans.

^{500BC 500 AD}

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Where did all the Germanics come from? There was basically a suction effect, pulling them into Roman-held western areas.

Their homeland appears to have been the western Baltic Sea coastal areas. They spent the next few centuries expanding into what were likely non-farming (so lightly-held) areas north of the Roman Empire back to the Black Sea. The victims of this movement were likely Thracian, Slavic, and/or Iranian tribes.

The Roman areas were relatively depopulated in this era. McEvedy and McGrath report that the Roman Empire's population dropped by about a fifth from its 2nd Century apex to the 4th, and then another 20% in the next two centuries (so about a third in total).

Whatever was going on, it didn't seem to hit the Germanics and Slavs as hard. As McEvedy put it:

For the population nadir coincides with the age of the Vikings and Varangians, and the intense activity in which these people engaged strongly suggests that, whereas there may have been less people in Europe as a whole than there had been 500 years earlier, there were a lot more Scandinavians and Russians.

Secondly, the Germanics during this same period had most of their territory conquered from the East. This left a large amount of Germanic tribes crowded into a very small amount of free Germanic territory, abutting a lot of recently depopulated Roman territory.

Below are small crops of Colin's maps for 362 and 451 AD respectively. The thick solid lines are Roman borders, the small dotted lines are Germanic borders, and the thick dotted Hun. The areas shaded with vertical lines are controlled by Celtic speakers.

Nature abhors a vacuum, so the result was probably inevitable: The Germanic tribes moved West.

Answer 2

A recurring theme of European history is the "drift" south--toward warmer climes. For instance, "Norsemen" left Norway and ended in Normandy, Swedes migrated to the south shore of the Baltic; Poles left the Baltic and headed toward the Balkans and the modern Ukraine, etc.

The Celts "started" (around 500 B.C.) in modern Poland, Austria, Germany, and France (known at that time as Gaul). Over the next thousand years, they drifted (mostly) south to Iberia, Galatia (in modern Turkey), parts of Greece, and also "Cisalpine Gaul," that is "Gaul" or northern Italy south of the Alps.

"North Germans" started in what we know as "Scandinavia. Some of them stayed there, but others crossed the seas to the south shores of the North and Baltic Seas. When Celts made their migration south, they created a vacuum in modern Germany, Austria, Poland, Netherlands, etc., that the North Germans could fill. (The Celts stayed in Gaul, and it was not until the middle of the first millennium that German tribes such as the Franks could penetrate into what we now call "France.")

Put another way, the "migration" of the Germanic languages followed the "migration of people." There was "some" (but relatively little) conversion of people from one language to another.