Countries are usually feminine, many cities are of masculine gender. There are tons of exceptions, of course. They're treated just like other nouns: for words of masculine gender with the null ending (Лондон, Харьков) you get the same null ending (Лондон, Харьков) in accusative, because cities are inanimate (according to the general rule). For countries, as they're usually of feminine gender (end with -а or -я), you get the usual -у (or -ю) ending, as you would get for other nouns (nom. полиция - acc. полицию; nom. Германия - acc. Германию). Some countries are not declinable at all, it's usually names that end in a vowel other than -а/-я: Перу, Конго, и т.д. I think most native speakers have no idea what gender such indeclinable names have. Usually, you don't really need to know it anyway, because there are only two cases when you need to know the gender: to have an adjective properly synchronized in the form with the noun; and it's also needed in the past tense (endings -л, -ла, -ло). "Adjective+name of a country" is a pretty rare combination anyway: instead of "Kongo is beatitful", you would rather say "Kongo is a beautiful country" where "beatiful" depends on "country". Same with the past tense, i don't think we say "country X did something"