In order for a government to make a successful claim of eminent domain, which two elements must it satisfy ?

I don't believe it has to satisfy two elements. We are talking about a power that the state or a national government has, and with it it can take private property for public use. Sometimes it can be delegated by the state to municipalities, government subdivisions etc., only when they are authorized by the legislature to exercise the functions of public character. That property is will usually be devoted to public or civic use or, in some cases, to economic development.

Hope this helps!

Both countries had to aprove annexation.