Here's a short story:

Yerevan (, ; , sometimes spelled Erevan) is the capital and largest city of Armenia as well as one of the world's oldest continuously inhabited cities. Situated along the Hrazdan River, Yerevan is the administrative, cultural, and industrial center of the country. It has been the capital since 1918, the thirteenth in the history of Armenia, and the seventh located in or around the Ararat plain. 

The history of Yerevan dates back to the 8th century BC, with the founding of the fortress of Erebuni in 782 BC by king Argishti I at the western extreme of the Ararat plain. Erebuni was "designed as a great administrative and religious centre, a fully royal capital." By the late ancient Armenian Kingdom, new capital cities were established and Yerevan declined in importance. Under Iranian and Russian rule, it was the center of the Erivan Khanate from 1736 to 1828 and the Erivan Governorate from 1850 to 1917, respectively. After World War I, Yerevan became the capital of the First Republic of Armenia as thousands of survivors of the Armenian Genocide in the Ottoman Empire arrived in the area. The city expanded rapidly during the 20th century as Armenia became part of the Soviet Union. In a few decades, Yerevan was transformed from a provincial town within the Russian Empire to Armenia's principal cultural, artistic, and industrial center, as well as becoming the seat of national government. (End of story)

Provide your best guess for the following question. Give ONLY the guess, no other words or explanation.

For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>

Here are a few examples of questions and answers:

Question: What is the capital of Armenia?
Guess: Yerevan

Question: What fortress was founded there in 792 BC?
Guess: the fortress of Erebuni

Question: Would this be noted as the begining of it's recorded history?
Guess: yes

Question: Wjem was ot tje Center of the Erivian Khanate?
Guess: 1736 to 1828

Question: What river is it situated near?
Guess: the Hrazdan River

Question: Is it the largest city in Armenia?
Guess: Yes

Now answer the following question in the same format.

Question: What county did it become part of in the 20th century when the City expanded rapidly?

the Soviet Union