Countries have artcles but lose them when introduced by in but get them back when modified some way:
Amo l'Italia (I love Italy)
Vado in Italia (I go to Italy)
Vado nell'Italia meridonale (I go to Southern Italy)
this detail was never explained earlier but we are expected to know it (?!) How?
By reading my comment. :-). But seriously you need a grammar and a dictionary - no way is Duo enough on its own. Also I think that you go in to large areas like countries and regions but a to smaller ones like towns.
You are very funny guy, Peter! Thank you for your wise advice!
Thank you, Peter, for the clear and concise rule.