How did the migrations of Germanic tribes affect the spread of West Germanic languages?

The migrations of Germanic tribes played a pivotal role in the geographic distribution and eventual fragmentation of West Germanic languages.

Beginning in the late 4th century, the great tribal migrations significantly altered the linguistic landscape of Europe:1

• Expansion and Settlement: North Sea Germanic speakers migrated south along the coast of the Low Countries and launched the conquest of Britain, establishing the foundations for what would become English. Meanwhile, the Rhine-Weser group (Franks) expanded into Gallo-Roman territories, and the Elbe group (including the Alemanni and Bavarians) moved toward the Alps.12
• Dialectal Fragmentation: These movements contributed to the breakdown of the Northwest Germanic dialect continuum. By the 3rd–6th centuries, this led to the emergence of three major divisions: North Germanic in Scandinavia, North Sea Germanic in Jutland and northwestern Germany, and South Germanic in central Germany.1
• Formation of West Germanic: The term "West Germanic" recognizes the strong affinities between North Sea Germanic and South Germanic, which shared specific linguistic innovations—such as the loss of final z in polysyllabic words and the gemination of consonants—that distinguished them from North Germanic.1
• Regional Shifts: In some areas, such as eastern Germany, the movement of tribes (driven by Hunnish incursions) led to the displacement of Germanic speakers, with the region subsequently settled by Slavic-speaking peoples for several centuries.3