I understood that nationality was based upon the country in which you were born.
The answer is ''not necessarily''. I was born in Australia to British parents in 1952 and therefore am Australian by right. That rule changed in the 1960s to prevent pregnant laydeez using it as an immigration backdoor and travelling to Oz to give birth.
Conversely, my parents failed to register my birth with the UK authorities. Without a UK birth certificate or naturalisation papers I now have endless trouble renewing my British passport. I've held British passports since about 1966, but the last time I renewed it I had to supply copies of my parents' birth certificates (b1907 and 1917) and marriage certificate (m1937). Troublesome when you live in Japan.