Cossacks
The Ukrainian Cossacks first rose as a warrior group of East Slavic Orthodox Christian horsemen who took up a military tradition and eventually created a Cossack Federation in the steppes of Ukraine.  They first rose up in the 15th century in response to the raids of Tatars and the conquests of the Ottoman Empire.  During Ottoman occupation of the western bank of Ukraine, the Cossacks rebelled against the Ottoman Empire and were responsible for destroying Ottoman influence in the region.  The Ottomans lost control over Ukraine and parts of Romania also due to the Cossack rebels and raiders. The Cossacks captured many Ottoman outposts in the Ukrainian steppes and created a Cossack Hetmanate (state). They sometimes served the Polish-Lithuanian Commonwealth forces and later fought against them, taking Ukraine.  Thus, the Cossacks are considered in Ukraine to be the founders of modern Ukraine since they expelled the Ottomans and Polish from Ukraine and created an independent state for themselves.