叛国 is a verb-object compound (离合词) meaning 'to betray one's country' or 'commit treason'. As a separable verb, the object 国 can be separated from the verb 叛 when inserting aspect markers (了, 过) or other modifiers between them — for example, 叛了国 ('betrayed the country') or 叛过国 ('has betrayed the country'). It is a formal, legal term used in contexts of national security and law.