1 {scene: An island off the coast of Antigua. The native cannibals have Captain Long Tom Short and Dirty Dirque stewing in a giant pot.}
1 Ponsonby: {bursting on to the scene} Release those pirates! They're now our prisoners!
1 Native: You and whose army?
2 Ponsonby: We're the Royal Navy! We don't need an army! We have muskets!
3 Native: We have hordes of silent, invisible warriors hiding in the bushes, armed with blowguns and curare-tipped darts.
4 [caption]: Ten minutes later... {Ponsonby and his Mate are now in the cooking pot}
4 Mate: I'm thinking an army would come in handy right about now, sir.

Curare is a potent toxin derived from various South American plants. The sources are mostly from the Amazon River basin region, but Antigua is close enough that they could probably get some imported.

---

2018-09-25 Rerun commentary: Looking up the idiom "You and whose army?" just now, I was surprised to find that some people also the use the variant "You and what army?" It's the first time I've ever heard that variant.

Anyway, the phrase (in both versions) seems to originate from American English, with one researcher finding an earliest printed example of "You and what army?" from 1929, in the Waterloo Evening Courier and Waterloo Daily Reporter^[1], from Waterloo, Indiana. This was followed quickly by an example of "You and whose army?" printed in 1930 in the St Louis Post-Dispatch from St Louis, Missouri.

I can say with some certainty that the "You and whose army?" version had spread into the lexicon of Australian schoolyard boys by the 1970s.

[1] Now that's a name for a newspaper.