Robert De Niro has dumped on Trump for years; why did POTUS wait so long to punch back?

If there's one thing we know about President Trump, he always punches back at his critics 10 times harder — and we have first lady Melania Trump to thank for that insight.

So it's curious the president waited so long to punch back at actor Robert De Niro, who's been slinging insults at Donald Trump since at least 2011 with nary a tweet about it from the Twitter-obsessed president.

Until now.

Headed back to Washington from Singapore on Air Force One following what he called "a truly amazing" summit with North Korea, Trump sent out two tweets jeering that the star of Raging Bull had taken too many hits to the head.

"Robert De Niro, a very Low IQ individual, has received to many shots to the head by real boxers in movies. I watched him last night and truly believe he may be “punch-drunk.” I guess he doesn’t...

...realize the economy is the best it’s ever been with employment being at an all time high, and many companies pouring back into our country. Wake up Punchy!," POTUS said in two tweets.

This followed De Niro's profane outburst at Trump during the Tony awards Sunday night, which was quickly bleeped by TV censors while many in the audience stood and cheered.

Then, at an event on Monday in Toronto, De Niro took it upon himself to apologize to Canadians for Trump's "idiotic behavior" in lobbing insults at Canadian Prime Minister Justin Trudeau since the G7 summit in Quebec ended in acrimony between Trump and the other member nations.

But a review of De Niro's history of deriding Trump — in public, on video, in speeches, in interviews, at press conferences — shows that "idiotic behavior" is the mildest insult De Niro has aimed at Trump over the years.

Trump has been relentless in attacking other Hollywood figures who have criticized him. Just ask Rosie O'Donnell, Meryl Streep, Arnold Schwarzenegger, Alec Baldwin and a host of others. But not De Niro.

So what made POTUS notice this time? Did he consider it especially lèse majesté after concluding what he believes is a successful deal to rein in North Korea's nuclear threat? Did he just grab his smartphone during down time on the long flight home?

Or did he just have enough shade from Bobby De Niro?

Because the actor, 74, has been searing on the subject of his fellow New Yorker. Only once has Trump responded and it wasn't in a tweet.

Here's a chronicle of De Niro's Trump slams:

In April 2011, in an interview with Brian Williams at his Tribeca Film Festival, De Niro compared Trump, without naming him, to a "car salesman" who promises a lot but can't back it up.

Trump later told Fox & Friends (still his go-to outlet) that he liked De Niro as an actor but he's "not the brightest bulb on the planet....In terms of when I watch him doing interviews and various other things, we're not dealing with Albert Einstein." Even then, Trump was using one of his go-to insults: diminishing his critic's intelligence.

Once Trump announced he was running for president, De Niro didn't hold back.

August 2016: "What he's been saying is really totally crazy, ridiculous ... he is totally nuts," De Niro said at a film festival in Sarajevo.

October 2016: De Niro appeared in a #VoteYourFuture campaign video ad in which he called Trump a series of insults.

“He’s a punk, he’s a dog, he’s a pig, he’s a con, a bull---- artist, a mutt who doesn’t know what he’s talking about, doesn’t do his homework, doesn’t care, thinks he’s gaming society, doesn’t pay his taxes,” he said. “He talks (about) how he wants to punch people in the face? Well, I’d like to punch him in the face.”

November 2016: After Trump was elected, De Niro described his shock in an interview with The Hollywood Reporter. "I feel like I did after 9/11," he said.

January 2017: De Niro appeared at an anti-Trump rally in New York before the inauguration to read parody Trump tweets. De Niro declared Trump is "a bad example of this country, this city."

February 2017: On The View, De Niro tells the panel he still wants to punch Trump in the face because bullies need to be confronted.

"It was only a symbolic thing," De Niro said. "But he’s got to hear it. He’s got to hear that, you know, that’s how he makes people feel. It’s not good to feel that way. It’s not good to start that stuff up, but at the same time sometimes when people are bullies like that, that’s what you have to do to shut them up. Bully them back.”

May 2017: In a commencement address at Brown University, De Niro says America has "turned into a tragic dumbass comedy" thanks to Trump.

August 2017: In an interview with Deadline, De Niro said if Trump were smart "he'd be even more dangerous. He's dangerous as it is. He's terrible, and a flat-out blatant racist and doubling down on that, and it's good that he does because he's going to sink himself."

January 2018: At a National Board of Review gala, De Niro went on a profanity-laced tirade against Trump, calling him an "idiot," a "fool," "our baby-in-chief" and "the jerk---in-chief." He said Trump has "put the press under siege, ridiculing it through trying to discredit it through outrageous attacks and lies."

WSBTV