At the start, Ankh-Morpork and Klatch are in diplomatic dispute over what to do about the newly-emerged island of Leshp, smack-dab in the middle of their waters.

We read the hum-drum, everyday racism and xenophobia through the mouths of Sergeant Colon and Corporal Nobby Nobbs

> “And o’course, they’re not the same color as what we are,” said Colon. “Well…as me, anyway,” he added…

> “Constable Visit’s pretty brown,” said Nobby. “I never seen him run away. If there’s a chance of giving someone a religious pamphlet ole Washpot’s after them like a terrier.”

> “Ah, but Omnians are more like us,” said Colon. [...] "Just the same as us underneath.

Never underestimate anyone's capacity to discriminate against foreigners over the tiniest things - especially Colon, the median voter in a 'one man, one vote' city state (👇):

> "…Whereas we’re more civilized, see, and we got a lot more stuff around to count, so we invented numbers. It’s like…well, they say the Klatchians invented astronomy—”
“Al-tronomy,” said Nobby helpfully.

When the Prince's life hangs in the balance, after an assassination attempt, Ankh-Morpork mobilises for war. Everyone suspects Klatch of a false-flag - everyone, that is, except for Vimes. He suspects his own fellow Morporkians first.

Only later, when Vimes confronts Ahmed in Klatch, do we see that the Klatchian caliphate and its people are 𝘯𝘰𝘵, in fact, noble savages. Its government is just as ruthless as any other, to assassinate one of its own statesmen to justify a war over some rock.

At once, the switch in Ahmed's character is a twofer: First, it's a callback to his 'foreign' persona in Ankh-Morpork (he knows the city as well as any student of the Assassin's Guild should).

This satirises how some folks view others through a blend of ignorance and hubris, armchair anthropologists who see foreigners as though they're some art exhibit to be studied.

Second, Vimes' reaction to realising he's been well and truly had satirises those of us who have liberal fingers on our pulse - to see others right, no matter who they are, or where they come from.