Nations become sovereign when they are recognizes as sovereign by other nations and international institutions. Other countries can also challenge the sovereign

Nations become sovereign when they are recognizes as sovereign by other nations and international institutions. Other countries can also challenge the sovereignty of others. "legally declare" implies some other outside legal power that decides it for once and all. There is no such thing. Not even the UN. For example, Taiwan is de facto sovereign controls it's borders, but not de jure. China challenges it. Only few nations recognize it. It has no seat in the UN.