The country was too poor to warrant the effort, and it was easier to maintain border security to protect the fertile lands than the cost and effort of occupying the whole territory.
It was such poor territory it wasn't worth the effort of taking it over and having the problems of administration, with insignificant reward.
By conquest.
The Byzantine and Persian empires were weak.
He added an area in Central Asia, Thrace and Macedonia.
His persistence in running a 10 year military campaign to take over all the Persian Empire.
They successively set out to expand and consolidate the Persian Empire.
550 BCE when Cyrus the Great began to establish and expand it to 431 BCE when Alexander the Great took it over as the Macedonian Empire.
Yes, much further. The Roman empire stretched, roughly, from northern Africa to Scotland and from the Atlantic Ocean to the Black Sea.
When it conquered and incorporated the Median Empire in the second half of the 6th Century BCE and was able to use the combined resources to expand to the east and west.
The Greek city-states in Asia Minor revolted against Persian rule. Cities outside the Persian Empire intervened, so the Persians decided to incorporate them within its empire to enforce peace.
It was the campaign by King of Kings Xerxes of Persia (Achaemenid dynasty), son of Darius the Great, who followed in the steps of his father and attacked the Western fronts of the Persian Empire to expand Persia and to conquer Greece.
What did Qin king use to expand his empire