This is the 167th most frequent Croatian word.
hrvatsku
"Croatia" (accusative case).
Here, 'hrvatsku' modifies the noun 'povijest' and agrees in case, gender, and number, functioning as an adjective meaning 'Croatian.'
Istražujem hrvatsku povijest u slobodno vrijeme.
I research Croatian history in my free time.
In this sentence, 'Hrvatsku' is the accusative case of 'Hrvatska' and is used as a direct object to indicate the country that is being loved.
Volim Hrvatsku jer ima lijepu obalu.
I love Croatia because it has a beautiful coastline.
In this example, 'Hrvatsku' is again in the accusative case, marking it as the direct object of the verb 'posjetiti.'