Fantasy authors are rarely as meticulous in their worldbuilding as JRR Tolkien. Thus fantasy countries are often borrowed from the history books, if not the current world map. For example, GRR Martin's Westeros is just the UK with some rotation.

"Overlord" by Kugane Maruyama is a fantasy isekai in which an introverted salaryman is reincarnated as a lich commanding a legion of monsters, whom he leads to genocide a country full of White Christians.

Nazarick is obviously revanchist Imperial Japan. Maruyama's resentment against Europeans is understandable, considering the USA's long history of dominating Japan.

In Overlord, some countries are majority human, whereas others are majority monster, e.g. minotaurs. The human countries often represent White countries. I can recognize a few just from their map and wiki descriptions:

• Re-Estize Kingdom = Australia, weak due to dependence on natural barriers, corrupt and divided, first White target for Japanese conquest
• Slane Theocracy = USA, aggressive high-tech theocratic republic with a racist history
• Baharuth Empire = Russia, with bloody tsar purging nobility.
• Roble Holy Kingdom = United Kingdom, complete with Hadrian's Wall and a queen. Abelion Hills = white cliffs of Dover = perfidious Albion = England.

The nearby lizard people are the Southeast Asians of Oceania whom Imperial Japan easily conquered.

I can't find China, probably because China is about to conquer Japan, and the author would prefer to reminisce about the glory days instead. It might be one or more of the "dragon" countries.

What others can you find?

Discussion

Why Australia?

• Proposed Japanese invasion of Australia during World War II | Wikipedia
• 1942 - An Overview of the Battle for Australia | ANZAC Day Commemoration Committee

The Japanese author might regard Australia as corrupt because it was founded by banished criminals and has much more murder than Japan. Or he might be criticizing the corruption and hypocrisy of the Anglosphere West in general, with Australia being merely the nearest example thereof. Japanese tend to be dismissive of Australia.

I explained the above to the r/Overlord subreddit. They didn't like it, and grew angry.

Overlord doesn't seem to be aimed at an audience with much awareness of geopolitics. Otherwise they might grow appalled at a Japanese man unsubtly glorifying the atrocities of the Japanese Imperial Army.