Last year, I was reading this Korean web novel where the MC returned from another dimension to Earth, only to find out that demons/devils (?) had invaded Earth. The last part of the story I remembered was when he protected a squad of soldiers from an unexpected onslaught of higher-level demons. Hope it is ongoing, I don't remember why I dropped it or forgot to continue reading it.
Guys it is Max Level Newbie. It is the right one. He had finally beaten the first dungeon and got sent home to find it being attacked by another race
That is true XD But I am fairly sure the one he wants would be Max Level Newbie. At least 80-90% sure. XD
I do remember a few KR novels with similar, but it depends on the particulars. The Returner has him first vanishing to another dimension / world, then returning and finding out portals are summoning monsters there and he 'joins' a government agency which protects the nation against the monsters. Then there's another one with the MC being a skilled fighter and in a sports store when suddenly monsters start appearing just about everywhere, and he helps a few people there before heading to a small military base where he effectively take control of the troops there to start hunting the monsters, it's in my list, but I can't remember the name. Seoul Station Necromancer's MC returns from another world and has the monsters confined mostly in dungeons and rarely heading out from them.
I think the sports store one is First Hunter. I haven’t read it in a long time so I can’t be sure. Also, for the one the OP is looking for, I have no idea.