Bro I'll be real with you north Korea got fucking obliterated to the extent that they were living underground because every single building has been bombed to rubble. North Korea was definitely the victim of imperialist interference. No person in that country deserved that level of devastation.
The other two flags are North Korea (left) and Serbia (right).
North Korea began the Korean War by invading South Korea. It didn't end well for them (or for the South, for that matter) but South Korea prospers today while the North is...well, North Korea.
For Serbia, it is one of the many states formed from the dissolution of Yugoslavia. Going from one country to several countries which were defined mainly by ethnic association was not a clean transition. There were Serbs living in other former Yugoslav countries, and Serbia used that justification to start occupying those majority-Serbian areas, similar to what Russia has been doing with Ukraine, Transnistria, South Ossetia, and to a lesser extent Abkhazia.
Ehhh the Korean war is complicated. It started with Japan invading Korea and attempting to erase their history and culture. (Worth noting that America gave them the green light on this). After WW2, Korea was divided and backed by US/USSR interests on either side. It was a shitshow with everyone disagreeing how things should be run. Eventually North Korea did invade the South, though, so I guess this meme still applies. But even that fact is unclear - they claim it was retaliatory. Who knows.