noun
permanently neutral country
A country that has declared permanent neutrality, recognized by international law, such as Switzerland.
スイスは永世中立国として知られている。
Switzerland is known as a permanently neutral country.
中立国 means a neutral country in a specific conflict or generally, while 永世中立国 specifically refers to a country with permanent neutrality status.
Compound of 永世 (eternal, perpetual) + 中立 (neutrality) + 国 (country).