My first guess is because they were attacked by foreign invaders.
(but at the same time, both empires had split in their own way. The Han empire lost control and broke into different areas. The Roman Empire had become two parts, where the Western part was constantly attacked by Germanic tribes). Both sides were constantly attacked by invaders until there was no more land to take.
Explanation:
Textbooks
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Biographical works
Reference books, including dictionaries, encyclopedias, and atlases
History books and other popular or scholarly books
Commentaries and treatises
Answer:
At the start of the twentieth century there were approximately 250,000 Native Americans in the USA – just 0.3 per cent of the population – most living on reservations where they exercised a limited degree of self-government.
Explanation: