Updated at 12:30 p.m. ET

Just moments after Air Force One lifted off for Buenos Aires, Argentina — where President Trump was to meet with Russian President Vladimir Putin — Trump canceled the meeting via tweet. He cited intensifying Russian aggression toward Ukraine.

Over the weekend, Russian vessels reportedly fired on Ukrainian ships in the Black Sea, seizing three vessels and injuring Ukrainian sailors. Putin has blamed Ukraine for the incident, accusing the Ukrainian president of orchestrating the whole incident for political gain.

U.S. officials all the way up to Secretary of State Mike Pompeo strongly condemned the Russian action. Yet even as Trump teased the idea of pulling out of the meeting with Putin on Wednesday, it had stayed on the schedule.

Less than an hour before tweeting that the meeting was off, Trump told reporters gathered on the South Lawn of the White House that "we haven't terminated that meeting. I was thinking about it, but we haven't," Trump said, adding, "I think it's a really good time to have the meeting."

Trump said he would be getting briefed on the Black Sea incident on Air Force One and would make a determination. Then, the tweets came.

"Based on the fact that the ships and sailors have not been returned to Ukraine from Russia, I have decided it would be best for all parties concerned to cancel my previously scheduled meeting........in Argentina with President Vladimir Putin. I look forward to a meaningful Summit again as soon as this situation is resolved!" Trump wrote.

The last time Trump met at length with Putin was over the summer at a summit in Helsinki, Finland. At a joint press conference at the end of the meeting, Trump deferred to Putin's denials of election interference and questioned U.S. intelligence. Putin came away grinning, and even Trump's supporters in Congress criticized his performance.

Thursday's cancellation comes as Trump heads to Argentina for the Group of 20, with a packed schedule and a number of looming questions that could determine whether the trip is a success or a mess. And that's not even to mention the new guilty plea from his longtime personal lawyer and fixer Michael Cohen that could overshadow it all.

Traditionally, these big international meetings have culminated with a communique that is agreed to by all or most of the nations in attendance. But Trump isn't a fan of multilateral, well, anything. So, his emphasis will be on a series of bilateral meetings on the sidelines of the gathering.

His dance card is in flux, but the White House says the president plans to meet with the leaders of Argentina, South Korea, Japan, Germany and India. And Trump will close out the trip with a working dinner with Chinese President Xi Jinping.

Without Putin on the schedule, the Xi meeting will get the most attention and raise the greatest uncertainty.

Deal or no deal?

When Trump sits down for dinner with Xi, trade will be on the menu. The United States and China are in the midst of a trade fight that is set to escalate Jan. 1. That's when the 10 percent tariffs the Trump administration imposed on Chinese goods are set to amp up to 25 percent. And if that were to happen, China would inevitably retaliate, as it already has for earlier U.S. tariff announcements.

On the South Lawn of the White House as he prepared to depart for the trip, Trump laid out his thinking about the possibility of a deal with China. "I think we're very close to doing something with China, but I don't know that I want to do it," Trump said. "I think China wants to make a deal. I'm open to making a deal. But frankly, I like the deal we have right now."

Earlier in the week, White House economic adviser Larry Kudlow told reporters that while Trump is optimistic, "certain conditions have to be met with respect to fairness."

Major sticking points include intellectual property theft, forced technology transfers, tariffs and other barriers to free and fair trade.

Trump and Xi have a good rapport, but it's not clear that will be nearly enough to resolve the escalating trade tensions. National security adviser John Bolton sought to temper expectations of a major breakthrough. He says the idea is that it's always good to get the two leaders together to exchange views, but there's not an expectation that "at this meeting, there'll be some substantial agreement coming out of it, but that there would be an indication, a kind of way ahead that the advisers could then pursue."

Possible scenarios for the Trump-Xi meeting:

This would likely include China agreeing to buy more from the U.S., perhaps liquefied natural gas and some general language about opening markets or working to resolve conflicts over intellectual property. This is the kind of "deal" that is far from done and could fall apart, but not until well after the initial positive headlines.

As Dollar puts it, "The key issue is: Is President Trump willing to take a practical compromise?"

It's unclear whether Trump's stated ambivalence about an agreement with China is a negotiating tactic or a firm resistance to such a compromise.

But what about the allies?

Trump has been a major X-factor at recent international gatherings, rebelling against the consensus or lecturing fellow world leaders about doing their part on trade or mutual defense.

Early in his administration, leaders of traditional U.S. ally nations like French President Emmanuel Macron worked to charm Trump into seeing things their way. But the charm offensive seems to have ended, having failed to keep Trump in the Paris climate accord or in the Iran nuclear deal, and with Trump imposing trade tariffs and threatening more.

"They will be expecting some instability because of the wild card factor that's at play," said Thomas Wright of the Brookings Institution. Whereas in the past, a lot of effort was put into crafting a communique nations could agree to, "I think they put a lot of time now into trying to minimize the damage from the summit."

In a letter to G-20 leaders setting the table for the meeting, United Nations Secretary General António Guterres laid out a series of challenges to be addressed, including climate change.

"More, rather than less, cooperation is needed," he wrote. "We must preserve and renew multilateralism to redirect globalization towards sustainable development for all."

That reads like a near direct contradiction of Trump's "America First" approach to foreign policy.

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