President Donald Trump had a busy weekend on his hands, with the arrival of Japanese Prime Minister Shinzo Abe taking up a significant portion of his last couple days. Hosting one of America's top allies on a diplomatic visit kept his attention off of his constant battle with the press—at least for a moment.

But following the prime minister's departure, Trump set his sights on another target this morning: Mavericks owner Mark Cuban. In a one-off Tweet sent shortly after announcing Abe was going home, Trump launched a seemingly random attack at Cuban's ability to run for president in the future:

In response to the Trump's Tweet, Cuban laughed and shared correspondence between the two from 2016, including an email from Cuban that questioned the judgment of the then-Republican nominee:

The (partial) contents of the 2016 email read as follows:

Nothing happened. Once you got past [Ted] Cruz I really hoped you would dig into the issues. You have a real chance to win. But to make it work you have to really dig in on the issues.

It's like the tech stuff today. The number of tech IPOs has fallen off a cliff. There are hardly any new public tech companies. So when you say there's a bubble, it sounds like you are winging it, which also makes it sound like your comments are based on who you talked to last. Which tells me you are getting bad advice.

I get that a big part of your base doesn't care about issue details, but to be President, to be this close, you have to dig in and know your sh*t. You don't have to bore people with details, but you have to learn the details.

Everyone else is afraid of you. I like to challenge you.

Cuban has never been shy about sharing his opinion with the masses, and has become one of the most recognizable owners in pro sports as a result. The Mavericks owner and Shark Tank co-panelist is a loud, brash thinker, and he was vocal during the 2016 election cycle about his preference for Democratic candidate Hillary Clinton. Cuban has been a thorn in Trump's side for months, and offered him $10 million last September if he'd agree to a sit-down interview with the NBA renegade. 

However, it was not immediately clear what Cuban had done or said to prompt this recent outburst from Trump. One possible theory is that Cuban fanned the flames of the Nordstrom controversy after the store told Ivanka Trump it would stop carrying her clothing line:

Not even Cuban knows what provoked the attack from President Trump. We'll keep you updated if further details emerge about this back-and-forth between a self-made billionaire and a reality television star.