How did Proto-Germanic develop into the West Germanic languages?

The development of the West Germanic languages from Proto-Germanic was a gradual process characterized by the emergence of dialectal groupings rather than abrupt splits.1

Initially, Proto-Germanic speakers occupied a region spanning southern Scandinavia to the North Sea and Baltic coasts. By roughly 250 BCE, this population had differentiated into five groups, including North Sea Germanic and Rhine-Weser Germanic. The first major linguistic division occurred between 1st–3rd centuries CE, separating East Germanic from Northwest Germanic.1

Between the 3rd and 6th centuries CE, Northwest Germanic fragmented into three major dialects: North Germanic, North Sea Germanic, and South Germanic. The term "West Germanic" was adopted to describe the strong linguistic affinities between North Sea Germanic and South Germanic, which shared specific innovations not found in North Germanic. Key developments defining the West Germanic group include:1

• The change of ð to d in all positions.1
• The replacement of inherited second-person singular past tense forms of strong verbs.1
• The gemination (doubling) of consonants preceded by a short vowel and followed by j (except r).1
• The loss of the final z in polysyllabic words.1