As a Brit interested in (but healthily critical of) Churchill, I've never heard this sentiment attributed to him before. I mention my nationality because you'd think our Falklands-touting media would jump at a chance to quote Churchill when making their arguments. But by all accounts, he was one of the warmer Prime Ministers we've had when it came to relations with Argentina.

Additionally, there is no reason why Argentina would be discussed at the Yalta conference — definitely not to the degree this quote suggests it was. Why would Churchill delve into South American politics at a conference focusing on Europe's future? Why would anyone there have thought Argentina (at this point in time) was in any position to become a world power regardless? And how could this particular quote alone enter public knowledge without the wider discussion surrounding it also being recorded?

With all this in mind, the most likely conclusion is that he never said this.

Was it not Brazil? The US apparently proposed at Yalta that the country should have a permanent UN Security Council seat (it had left the League of Nations in protest at being denied one), but Britain and the USSR were against. I've no idea as to the accuracy of the quote, but the time and place seem right even if it's the wrong country. Argentina certainly wasn't a candidate for membership of the Allied club at the time, having maintained neutrality and been suspected of pro-Axis sympathies, whereas Brazil had joined the Allies in 1942.