Desmond is wrong, and Widmore knows it by Ben

In Happily Ever After, when Desmond awakens from his exposure to the flash-sideways. Widmore is ready to explain what just happened: “I'm really sorry we had to do this to you, Desmond. But, as I told you, your talent is vital to our mission. So, if you just let me explain...”

Desmond brushes him off, saying that he understands what must be done, and Widmore is visibly surprised (“What”?).

The thing is that Desmond has got it all wrong – he believes that their current timeline – the one where Oceanic Flight 815 crashes on the island, is the hollow and meaningless one.
That is why he was not scared of FLocke (LOCKE: Why aren't you afraid?, DESMOND: What is the point in being afraid?) and of being shot by Sayid.
He believes that he will still be alive in the Flash-Sideways and that that is all that matters.

Widmore was anticipating that Desmond would think that the flash-sideways was “false” and was ready to convince him otherwise. Luckily for him, Desmond got it all wrong – and readily embraced the Flash-Sideways.

Widmore knew that the only way Desmond would be convinced into making a sacrifice (“But once it's over I'm going to ask you to make a sacrifice. And I hope for all our sakes you'll help me”) was by believing that no matter what happened, he would still be alive in a happier place (timeline) with Penny, and the hope for a child.

(At this point I am assuming that the sacrifice means dying to kill the Man in Black.)

So Desmond will march towards the Man in Black, ready to sacrifice himself. It will too late for him when he realizes that his current timeline is the “real” one – one where he already has a loving wife and child.