Complex sentence: The eastern island-group became a territory of the United States (the Tutuila Islands in 1900 and officially Manu'a in 1904) and became known as American Samoa; the western islands, by far the greater landmass, became known as German Samoa.
Split into simpler sentences: The eastern island-group became a territory of the United States (the Tutuila Islands in 1900 and officially Manu'a in 1904) and was known as American Samoa. The western islands, by far the greater landmass, became German Samoa.