I wasn't able to catch the entirety of former Secretary of State Hillary Clinton's speech prosecuting the foreign policy case against former reality TV star Donald Trump, but what I did see was, I thought, especially strong. Not only did she skewer and damn the presumptive Republican nominee, but she did so in a way that made people laugh at him.

One section especially rang true for me: "This is not someone who should ever have the nuclear codes," she said, "because it's not hard to imagine Donald Trump leading us into a war just because somebody got under his very thin skin."

It's a great point and could only have been made more effectively if she'd had the footage of the "Daisy" ad running behind her.

The signature moment of Trump's ridiculously thin skin remains for me his ware-hawking, Trump-brand infomercial "press conference" back in March. You remember it: He made a post-primary media appearance surrounded by what he purported (falsely, it turned out) to be various Trump products – steaks, a magazine, wine and so on – at a time when he was making the first of his many attempts at seeming presidential. The performance was a truly bizarre moment even by the standards of this campaign.

Editorial Cartoons on Donald Trump View All 455 Images

What is less well-remembered is what spurred it: Several days before, 2012 GOP nominee Mitt Romney had given a speech lambasting Trump as a fraud and a con-man and used his various failed brands as examples to make his case. The speech got some attention but had little impact – except with Trump.

And while much of any Trump appearance is off-the-cuff and meandering by its nature, Tuesday night's sales pitch was not. He had the props collected and displayed beforehand. This wasn't impromptu – he consciously decided to interrupt his regularly scheduled impersonation of a president in order to very unpresidentially hawk his wares, because he couldn't stand that someone had questioned his brand, even if that someone was an establishment loser and the questioning had obviously gotten no traction.

So thank you Mitt Romney, for getting inside The Donald's head. I imagine it's a terrifying place to be, but you have blazed a trail for others to follow.

God forbid those others should be some hapless foreign leader who insults Trump – or, worse, some smart foreign leader who skillfully pushes Trump's buttons.

Trump famously likes to describe himself as a "counter-puncher" – someone who hits back, harder, when he is attacked. He's obviously proud of the image but another way of seeing it is as someone who cedes the initiative to his opponent and someone whose self-image dangerously limits his range of possible reactions.

Of course, the attack does double duty: It illustrates Trump as a dangerous president and it also pricks that famously fragile crust. Trump, predictably, reacted the only way he knows how: ad-hominem Twitter attacks utterly devoid of substance. This is why Twitter is his favored medium, because he can avoid substance with it, which is good because he's unable to conjure any. (Recall what a fiasco Trump's attempt at a foreign policy speech was.) And his reaction also revealed the trap Clinton had laid for him. By responding, he confirmed that he's thin-skinned; if he had ignored it, though, he would have looked weak.