Prince Paul submitted to German pressure and signed the Tripartite Treaty, it was an attempt to keep Yugoslavia out of the war but it was hugely opposed by the people, a successful coup d'etat was launched two days later and King Peter was given full powers and Yugoslavia withdrew from the Treaty.
Two weeks later - Germany, Italy and Hungary invaded Yugoslavia, overrunning it within eleven days.
The beleaguered country was then split up amongst the Axis powers - the Independent State of Croatia had been established in 1929 as a Nazi puppet state but it had being very limited in its activities until now; German troops also occupied Bosnia and Herzegovina as well as parts of Serbia and Slovenia; other parts of the country were occupied by Bulgaria, Hungary and Italy.
The Yugoslav resistance that opposed the occupation consisted of two groups - the communist led Yugoslav Partisans and the royalist Chetniks.
The former were led by Josip Broz Tito and possessed pan-Yugoslav leanings while the latter were led by Draza Mihaljovic and were pro-Serbian.
The Chetniks were initially supported by the exiled royal government and the Allies but they concentrated more on fighting the Partisans and began to build links with the occupying forces that they were supposed to be fighting, by the end of the war they had evolved into a fully collaborationist Serb nationalist militia.
On the other hand, the Partisans developed from a small guerrilla force into largest resistance movement in Europe achieving notable successes against the Axis powers.
They expelled the Axis powers from Serbia in 1944 and the rest of Yugoslavia in 1945.