亡国 (wáng guó) is a verb-object separable compound (离合词) meaning 'to lose national independence' or 'to be conquered as a nation'. You can split it by inserting aspect particles, measure words, or other elements between the syllables, for example 亡了国 ('lost the country') or 亡过一次国 ('has been conquered once'). It can also function as a noun meaning 'conquered nation', but its core usage is verbal. When you want to say a country was overtaken, you can use 亡国: 那个国家亡国了。