This is maybe more of just a hypothetical or unimportant, but given the current situation in Ukraine maybe it could illuminate the correct position there.

Anyway, generally anti-imperialists have 2 rules

1.Fight against imperialist wars, even if it means your side will lose, to further revolution and stop the useless death and destruction

2.Support the self determination of people's in order to further the national revolution (that will lead to further socialist revolution down the line, and prevent the expansion in the labor aristocracy of the imperialist nation)

But the problem for people in Belgium and Serbia during this time is that one is conflicting with the other. If you fought to get self determination for Belgium and Serbia, you would be supporting inter-imperialist war. If you fought to hamper the war effort in these nations, you would be cosigning them to be subjugated (although thus perhaps applies more to Serbia than too Belgium. But I bring Belgium up since initially they were neutral in the war and only got involved because of the German invasion, unless I'm wrong on that of course)

Again, this is isn't directly pertinent to the modern day, save more maybe some comparison to current geopolitical events, but I'd appreciate answers anyway