Simplify the following text for a Grade 12 native English-speaking person.
Original: Hitler and Mussolini installed the Croatian UstaÅ e , who had long sought Croatian independence , forming the Independent State of Croatia ( NDH - Nezavisna DrÅ 3\/4 ava Hrvatska ) .
Simplified: Slavko Kvaternik , deputy leader of the UstaÅ e proclaimed the establishment of the Independent State of Croatia ( NDH - Nezavisna DrÅ 3\/4 ava Hrvatska ) on April 10 , 1941 .