Where is Nationalist China located?

Taiwan, this is the last remaining part of the faction called "Nationalist China" or "The Republic of China" which once controlled or contestedt most of mainland China before and during WW2.

It opposed "Communist China" during the Chinese Civil War (which started before WW2 had a half time break to fight the Japanese and resumed afterwards) and lost despite having the tacit support of the US and other western powers.

This is why modern China, ruled by the victorious communist faction, considers Taiwan a breakaway provence and Taiwan officially considers itself the true government of China.