caught the U.S. on the back foot = caught up the U.S.?

The US government was not expecting Australia to make such an announcement.
This expression "caught on the back foot" is probably from boxing or a similar combative reference point. The point is that if you are "on the back foot" you are not prepared either for attack or defence, you are unstable, you are at a serious competitive disadvantage.

Never heard the expression before, so I googled it. You'll be happy to know, dadpad, that your reply came up as something like the fourth hit--pretty high ranking. It seems to have something to do with cricket, since it comes up mostly in (ex)British Commonwealth countries and in descriptions of cricket matches (and I have always found cricket a pretty incomprehensible sport), and they seem to use the term without explaining it, so are there any cricket experts out there?

Maybe analagously, I was thinking of our own incomprehensible sport of baseball, where someone on the Red Sox tried to spin out of the way of an errant pitch and was hit on his back foot a couple games ago--maybe it means something you're not prepared for that surprises you unpleasantly.

MJ is right; "caught on the back foot" is pretty common in British English. Many dynamic sports require good foot control. In cricket, a high quick ball would force the batsman backwards to defend his wicket, and as he stands sidways, a right handed batsman would have his left foot nearer the bowler, consequently his right foot is his back foot. In boxing, "footwork" is important to preserve balance and avoid falling, and in tennis, too, stance and foot positioning matter. If you're moving forward, you're attacking. If you're moving backward (on the back foot) you're on the defensive (importantly, having been forced there by your opponent).