By going to war with and winning wars with other tribe/countries/empires.....and then incorporating the defeated into his domain.
They gained power by taking over Hittites empire
Greece and the Persian Empire.
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They were subsidiary to Media and took it over, then used this combined power to create their empire.
To gain religious and economical power. Also to gain wealth from the trade routes.
Imperialism means the desire of a country or government to gain imperial power, power through empire and gaining land.
To gain power octovian had to defeat jealous rivals
Spain
Cotton tobacco spices vegetables power, slaves to man their ships.
He extended Macedonian control throughout Greece and planned takeover of the Persian Empire. His assassination left the latter to be done by his son and successor Alexander.
look for the it yourself in the book
The attack was no surprise - the two armies were confronting each other. Alexander wanted to defeat the Persian army to gain control of the Persian empire.