I know this is a bit offtopic, but shouldn't the title say "the President of the United States," since 1) Churchill is UK's Prime Minister, and Roosevelt is not his President and 2) the publication on which this article appears is from the UK?

If the article itself (seemingly from the UK, as you point out) is inclined to assume that it's fine to describe it this way, I don't see why those of us from the rest of the world (which I'm inferring from your username is a group you're a part of) should object.

Even the original UK article just says "the President". Things may change as the US may not play the leading role in this century as it did the last, but despite many countries having presidents (although not the UK), it is still understood that "the President" means POTUS.

The UK doesn't have a president, so it normally refers to the US president in general context.

I think historic context provides enough clarity for readers of the website of the imperial war museum.

I also think the headline as is would be good enough even for people who might not have learned about ww2 yet. A headline is not meant to be a complete summary after all...