Was nihonshu (what English refers to as "sake") always called nihonshu, or was it only called that once western alcoholic beverages were introduced to Japan?
The 日本国語大辞典's earliest cite for 日本酒 (nihonshu) is 1886, in Tsubouchi Shoyo's 内地雑居 未来之夢, which is to judge from the title a book about foreigners in Japan. I'd say the chances are good that 日本酒 (nihonshu) is a recent coinage, and before that, the drink was simply called 酒 (sake).
Incidentally, again according to the 日本国語大辞典, the word 葡萄酒 (budōshu) for "[grape] wine" has been around for at least 500 years.
As mentioned above, all alcohol in Japan is 酒 (sake). When it is completely unambiguous, that word is sufficient to convey which form of alcohol you desire.
If you go into a London pub and say, "Give me a drink," you will likely get a pint of bitter. If you go into an inn in Dublin, you may just as likely get a shot of whisky. If you want to be unambiguous, you specify what you want.
For the record, in decades of visiting and living in Japan, I have never heard anybody order "sake" in a restaurant or pub. On the other hand, "sake" is often used to represent all alcohol, as in a store: "酒はどこですか。", or "Where is your alcohol?"