When the combined forces of Italy, Germany and Hungary invaded Yugoslavia in April 1941 the entire country fell in the space of less than two weeks with only a few thousand Axis casualties (mostly Italian, German casualties do not even amount to 1000 men). However, Yugoslavia had an infamously effective resistance movement afterward, with Axis losses, especially Germans, being many tens of thousands higher than during the invasion proper, and large parts of the country essentially falling out of Axis control due to the effectiveness of various partisan groups. Its said that Yugoslavia essentially liberated itself without need for direct intervention of other allied forces, all this despite major divisions within the country and significant collaboration. What was going on that made a country that seemed to be such a pushover in 1941 such a resistance powerhouse afterward?
Kingdom of Yugoslavia was defeated in the 1941 April War, mostly due to Royal Yugoslav Army having outdated weapons and tactics, and poor logistics. One story I distinctly remember is of a column of Yugoslav armored vehicles (mostly armored Škodas and some FT-35 tanks) leaving Belgrade under orders to connect with other units who had gas. The column somehow avoided Luftwaffe air raids, but when they reached their destination, there was simply no other unit there and no gas. There were units who fought with extraordinary valor (the Yugoslav air force defended Belgrade until the last of the Yugoslav pilots was shot down and by official accounts, they shot down around 40 German planes), and there were individual acts of courage (like the two naval officers who blew up their own vessel to block Italians from entering Boka Kotorska gulf).
As for the post-April War resistance, it is a complex story. For one, I believe the crimes perpetrated by the Nazis and their collaborators helped the resistance gain momentum and recruit people.
Most infamously, the Croatian Ustashe regime, installed by the Nazis in parts of Yugoslavia where Croats lived, massacred ethnic Serbs throughout their puppet state, and organized several death camps for Serbs, Jews and Roma (Jasenovac, Stara Gradiška, Kerestinec) and even a camp for Serbian children (Jastrebarsko)! These camps were established and manned by Ustashes themselves. The local Serb population had little choice but to join the resistance. Most of them joined the communist partisans, the rest joined the royalist chetniks (more on that later). As for the Croats who opposed to the Ustashe policy, they were persecuted and sent to the same camps, however, they were treated slightly better. Pre-war Croatian communists joined the local Serbs and formed the partisan units.
In Serbia, Nazis installed a weak puppet regime which tried to save Serbs running away from Ustashe-held areas, but did nothing to protect Jewish citizens from being rounded up and sent to camps. Mind you, Nazis were distrustful of Serbs and they deliberately chose to install a weak regime there, unlike in Croatia. In Serbia, Wehrmacht did the dirty work themselves and organized punitive actions against local Serb population - 100 Serbs were to be killed for 1 dead German soldier (most infamously, Wehrmacht lined up and shot hundreds of shool children in Kragujevac as retaliation for a deadly partisan attack). Waffen SS established death camps in Serbia in Belgrade (Banjica, Staro Sajmište) and in Niš. Northern Serbia on the other hand, was given to Hungary and Hungarian fascists rounded up a thousand of local Serbs and Jews in Novi Sad, shot them and threw them into the Danube river.
Macedonia was given to the Bulgarians who began an aggressive assimiliation campaign against the local Macedonian population. Macedonian language was forbidden and those who opposed were executed. Something similar happened in Slovenia, which was annexed by Germany. Slovenian language was replaced by German, however, with waay less persecution and fervor. In Bosnia, some Muslims joined the Ustashe (the Ustashe claimed that the Bosnian Muslims were "Croats gone astray") or the newly formed SS Handžar division.
As expected, people were standing in lines to join the resistance. Now, the only question is: which one? As the veterans in Serbia usually say, the communist partisans seemed well organized and disciplined, whereas chetniks looked more like a ragtag bunch of misfits. In reality, chetnik central command lacked vision and exercised little authority over their local commanders. Many local chetnik commanders had their own politics and many perpetrated crimes. On the other hand, the partisans were highly disciplined and motivated. In the beginning, both resistance movements accepted all nationalities, however, as time passed by, chetniks gradually began to profile themselves as exclusively Serbian nationalists. In the early years, the partisans and the chetniks even fought together, but after the battle for Kraljevo, they split ways and began fighting each other. The chetniks soon contacted the Germans and offered truce and some form of cooperation in fighting the communists.
So, as of 1942, the only true resistance were the partisans. Filled with Yugoslav veterans of the Spanish Civil War and of the First World War, backed by Moscow, later also supported by London, they were a highly succesful guerilla force.
TL;DR Poor logistics and outdated strategy/tactics crippled the Royal Yugoslav Army in 1941, despite acts of bravery. The resistance was succesful due to atrocities committed by Nazis and their helpers.
Edit: wording
Edit #2: Thanks for the Silver, kind stranger!
/u/commiespaceinvader knows a lot about this part of WWII: