This is the 88th most frequent Romanian word.
rusiei
"Rusiei" is the genitive/dative form of "Rusia," meaning "of Russia" or "to Russia" in English.
In this sentence, 'Rusiei' is used as the genitive case form of 'Rusia' (Russia), denoting possession ('capital of Russia').
Capitala Rusiei este Moscova.
The capital of Russia is Moscow.
Here, 'Rusiei' functions as a genitive modifier to 'harta' (the map), indicating the subject of the map.
Harta Rusiei arată granițele acestei țări mari.
The map of Russia shows the borders of this large country.
In this context, 'Rusiei' is in the genitive case used to specify whose history is being described.