When it was founded in 1929 by Ante Pavelić, the Ustaše was a nationalist organization that sought to create an independent Croatian state. It engaged in terrorist activity before World War II and ruled, under Nazi protection, in a part of Yugoslavia after that country was occupied by the Axis powers. It was responsible for the deaths of hundreds of thousands of people, particularly Serb.
After World War II, the Ustaša movement was split into several organizations and there is presently no political or paramilitary movement that claims its legacy as their "successor". The term "ustaše" is today used as (derogatory) term for Croatian ultranationalism.
Harry Turtledove sometimes uses the alternative spellings "Ustasha" and/or "Ustashi".
Ustaše in In the Presence of Mine Enemies
Croatia, under Ustasha rule, retained close ties with the Greater German Reich well into the 21st century. The Ustasha maintained its policy of persecuting Serbs with a zeal that made even the Gestapo found frightening.
Ustaše in "Ready for the Fatherland"
The Ustashi remained in power in the Independent State of Croatia under the wing of Germany, which established itself as the dominant power on Continental Europe after the peace treaty with the Soviet Union in 1943.
Ustaše in Supervolcano
One of the reasons Bronislav Nedic hated Croatians so much was the way they had jumped into bed with Hitler during the war and had used their irregulars, the Ustasha, to murder any non-Croats they could reach. Further, the Croats continued to have the Ustasha to the present day which Nedic had fought in eastern Slavonia while attempting to regain land stolen from Serbia during the break-up of Yugoslavia.