In the book 1984, Winston and Julia are at Victory Square and watch a convoy of trucks passing. What is their primary cargo?

What Julia and Winston and the crowds packed into Victory Square see passing by in the trucks are prisoners of war captured by Oceania in a recent battle. As the novel puts it,

In the trucks little yellow men in shabby greenish uniforms were squatting, jammed close together. Their sad, Mongolian faces gazed out over the sides of the trucks utterly incurious. Occasionally when a truck jolted there was a clank-clank of metal: all the prisoners were wearing leg-irons...

As we know from the novel, Oceania is perpetually at war with one of the two other superpowers, Eurasia or Eastasia. Since both these other superpowers seem to be Asian, the ethnicity of the prisoners gives no clue as to what country Oceania is currently fighting with--and it doesn't really matter, because the country changes all the time, and whichever country it currently is, the regime insists that country has always been the enemy. But through much of the first part of the novel, Oceania is at war with Eurasia.

We can surmise that these prisoners will be executed because while rewriting a news story, Winston creates a Comrade Ogilvy who creates a hand grenade that kills 31 Eurasian prisoners in one burst.