Will fight for the

What is the exact meaning of the sentence in bold? It is from the book “Persian Gulf Command” by Ashley Jackson, p. 179. It is a report about the Iranian attitude about the Allied forces who occupied Iran in 1941. Who is the subject of the sentence in bold? Why does not have it a mentioned subject?

Regarding the sentiments of Iranian officers in the Kermanshah and Hamadan regions, the consul reported:

General attitude summed up as We hate all foreign troops – be it English, Russian or even German. We like them as individuals apart from the Russians whom we all loathe. Will fight for the British against Russians anytime but against Germans can’t yet say. They doubted that the British would help them get the Russians out of Azerbaijan.

Thanks a million.