On 6 April 1941, the Axis powers invaded and quickly occupied Yugoslavia. Parts of the country were annexed by its neighbours, and the Independent State of Croatia ("Nezavisna država Hrvatska", NDH) was carved out as a Ustaše-ruled Axis puppet state. With the country's defeat imminent, the Communist Party of Yugoslavia ("Komunistička partija Jugoslavije", KPJ) instructed its 8,000 members to stockpile weapons in anticipation of armed resistance. By the end of 1941, the armed resistance spread to all areas of the country except Macedonia. Building experience in clandestine operation across the country, the KPJ proceeded to organise the Yugoslav Partisans as resistance fighters led by Josip Broz Tito. The KPJ assessed the German invasion of the Soviet Union had created favourable conditions for an uprising and in response to this, the KPJ politburo founded the Supreme Headquarters of the National Liberation Army of Yugoslavia with Tito as commander-in-chief on 27 June 1941. According to Yugoslav sources, the Partisan forces grew to 800,000 by 1945 through the recruitment of volunteers and defecting Axis-collaborator troops who were promised amnesty, and the conscription of men aged between 17 and 50.