This cheat will annex the country with the specified tag for either your own country or a specified country.
annex [country tag] [country tag]
|Country Tag||The country tag of the country you wish to annex.|
|Country Tag||Optional. The country tag of the country you wish to annex the first specified country. If you do not specify a country tag here, the first specified country tag will be annexed for the country you are currently playing as.|
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annex RUS CYP
The above command would make Cyprus annex Russia - meaning Cyprus would claim all of Russia's land. See all country tags here.
This cheat would make the country you are currently playing as annex Greece.