Question

What territories did Cyrus's successors add to the Persian Empire?

Answer 1

Cyrus the Great of Persia who founded the Achaemenid Empire embraced all the previous civilized states of the early Near East which expanded vastly and eventually conquered most of the Southwest Asia, much of Central Asia and Caucasus. Under his successors, the empire stretched from parts of Balkans and Thrace-Macedonia in the west to the Indus Valley in the East. 
Cyrus the Great had created the largest empire the world had ever seen.

Hope I helped:P