According to the rules about Baggage on page 9:
Baggage of an ally that could change sides must, and some or all of other generals can, remain with him as Command Baggage. All other baggage is held in an extra Army Baggage command...
Okay, so we go to Unreliable Allies on page 14:
If any of his side's commands became broken last bound and no enemy command is broken, either - If either he is irregular and of a different nation, or the two armies are of the same nation, existed at the same period and their army list specifies them an possible enemies, then his command will change sides and become a reliable enemy ally command.
As I read these points, it occurs to me that it's possible an army list I create could become retrospectively illegal once I see what my opponent's list is. That is, say I create a Sub-Roman British army with three ally generals. Although the generals are allies, they're of the same nation, so changing sides on that basis is impossible. So I create a list which puts all their baggage in an army baggage command. But then I find out my opponent has also taken a Sub-Roman British army "at the same period". Sub-Roman British include themselves among their enemies. So now my unreliable allies could change sides. So now my army baggage is illegal.
Or, to put it another way, if I enter a competition and know for certain that no one else will have a Sub-Roman British army, I can use army baggage.
After my last couple of efforts, I'm sure I'm missing something. What is it?