Ever since I first read the slogan "Hasta la victoria siempre" I always assumed that siempre was an adjective qualifying victoria. So if I had had to translate it into English I would have said something like "Towards eternal victory". But on looking in the dictionary of the language for something else I find that all the definitions there mark it as an adverb. This seems strange to me as I would have expected if it was modifying the preposition hasta it would have come first in the sentence. So is the RAE behind the times or is this just a stylistic thing and it really is modifying hasta?
Siempre is always an adverb.
The sentence would be translated as "always onward to victory", and it means that one should always pursue victory.
However, it was written in a letter that Ernesto Guevara (el Ché) wrote to Fidel Castro, and some people say that what was actually written was: "Hasta la victoria. Siempre, patria o muerte". In this case, siempre would also be an adverb and would also mean always.
Here is an image of the letter's manuscript:
In spanish you can have multiple orders and sentences structures, like when in english you say "I also want..." and "I want that too" but in spanish you can say, "siempre hasta la victoria" or "hasta la victoria siempre" it is normal and just a different way to say the same thing but the second one sounds better (more elegant).