On April 6th, 1941, Germany launched a massive assault on Yugoslavia. Within 12 days of the assault Yugoslavia was crushed. Four days after the German assault, on April 10th, 1941, Slavko Kvaternik came forward from the region of Croatia and proclaimed "a free and independent State of Croatia" under the direction of Ante Pavelic. At the time of the German assault and later during Kvaternik's declaration, Pavelic was in Italy. He arrived in Croatia on April 14th and took up control of the newly formed independent State of Croatia. On April 17th, Croatia declared war on the British, thus making Croatia a formal Axis partner.
Over the course of the Second World War many Croatians choose not to serve in the various branches of the Armed Forces of the Independant State of Croatia, but instead volunteered for service in one of the military forces of Croatia's Axis allies, namely with Germany, or to a lesser degree, with Italy. Croatians served in all branches of the German Wehrmacht the Waffen SS and the SS Police. The following listings is of the units and formations known to have been made up of Croatian volunteers in the service of the German Armed Forces: (Many Croatians served within other units on an individual basis, but their numbers and exact stories are not known to history due to the nature of their individual service.)
369th Reinforced Infantry Regiment
369th "Devil's" Division
373rd "Tiger" Division
392nd "Blue" Division Croatian Airforce Legion
Croatian Naval Legion
13th Waffen SS Mountain Division "Handschar"
23rd Waffen SS Mountain Division "Kama"
"Croatia" Police Regiment 1
"Croatia" Police Regiment 2
"Croatia" Police Regiment 3
"Croatia" Police Regiment 4
"Croatia" Police Regiment 5
Police Anti-Tank Company "Croatia"
Gendarmerie Division "Croatia"
Light Transport Brigade
Italian Croatian Legion both served under Italy during WWII.