Short answers, pulled from the story.
The Kingdom of Yugoslavia fell on the 6th of April 1941 when German, Italian, and Hungarian forces invaded from all sides. Belgrade was bombed by the Luftwaffe in a campaign that lasted little more than ten days before the Royal Yugoslav Army surrendered unconditionally on the 17th of April.
Josip Broz Tito chaired the Supreme Staff of the National Liberation Partisan Units of Yugoslavia which was established on the 4th of July 1941. The first liberated area known as the Miners Republic lasted 42 days in Stanulović mountain village under his leadership.
In November 1941, Chetnik leader General Mihailović turned against Partisans as his main enemy claiming humanitarian reasons for preventing German reprisals against Serbs. British liaison advised London to stop supplying Chetniks after the Užice attack yet Britain continued providing support through 1942.
By late 1943, the National Liberation Army was recognized as allied army at Tehran Conference for first time by all three Allied sides including United States. Brigadier General Fitzroy Maclean was parachuted to Tito's headquarters near Drvar on the 25th of September 1943 to serve as permanent formal liaison to Partisans.
Yugoslav government estimated casualties at 1,704,000 submitted figure to International Reparations Commission in 1946 without documentation while later study found total number killed was 597,323. Non-combat victims included majority of country's Jewish population who perished in concentration camps like Jasenovac located some 100 km southeast of Zagreb.