There were few bigger boogeymen on Donald Trump's campaign trail than China. He said the word so often and so distinctively that it invited mashups. He said the Chinese were taking advantage of us because we were stupid. He promised to label them a currency manipulator.

And now that he's president, basically no country has been so alternately on Trump's good and bad sides. From even before he took office, he's been sending hugely mixed signals about this relationship he wants with Beijing. Whether Trump is flying by the seat of his pants or is trying to be so unpredictable that other countries fear him, China is Case Study No. 1 in his erratic tendencies. And it continued Wednesday, with Trump complaining about China's trade with North Korea.

Here's a quick summary of the back-and-forth over Trump's first six months.

The one-China policy/flirting with Taiwan

In December, before he was sworn in, Trump broke nearly four decades of U.S. diplomatic protocol by holding a phone call with the leader of Taiwan, which is considered a province of China under the U.S.'s one-China policy. It seemed like a gaffe.

Soon, though, those who were involved in the call put out word that Trump was actually trying to keep China on its toes — that it wasn't an oversight at all and that he was briefed beforehand about the significance of that call. The Post reported that the call “was the product of months of quiet preparations and deliberations among Trump’s advisers about a new strategy for engagement with Taiwan that began even before he became the Republican presidential nominee, according to people involved in or briefed on the talks.”

The provocation apparently continued come inauguration time, when China objected to a delegation from Taiwan led by its ex-premier being allowed to attend and said it would hurt U.S.-China relations. The delegation still attended.

But whatever that strategy was, Trump pretty quickly abandoned it. During an early-February phone call with Chinese President Xi Jinping, he seemed to back off the provocation and assured Xi that the one-China policy was still the U.S.'s policy. It's not clear that he got anything in return, and experts suggested Xi had won the showdown.

And one final, recent development: Last week, the issue flared again, with the Trump administration approving a $1.4 billion arms deal with the Taiwan. It is not the first arms deal between the U.S. and Taiwan, but it came at an inauspicious time in the U.S.-China relationship.

Currency manipulators

Next to the border wall and talking about Hillary Clinton's alleged crimes, one of Candidate Trump's biggest applause lines was saying he would label China a currency manipulator. That continued through the end of his campaign, with his 100-day plan released shortly before the election including that plank prominently. He said that on his first day in office, “I will direct the secretary of the treasury to label China a currency manipulator.”

Then his first day came and went. Then some more days came and went. By April 12, Trump said he had decided against labeling China a currency manipulator, as he had promised dozens and dozens of times.

The reason? Trump said, rather remarkably, that it was in part because China suddenly stopped manipulating currency the day he was elected. Trump also said, equally remarkably, that he didn't want to alienate a partner in containing North Korea. (Nevermind, of course, that China has long been key to this U.S. effort, and that this fact didn't stop Trump from repeatedly making this promise during the campaign).

By April 16, though, Trump seemed to suggest the currency manipulator play might still be on the table, tweeting “We will see what happens!” — an admonition that will be familiar to one James B. Comey.

North Korea

Trump has repeatedly invoked China's central role when it comes to North Korea. But even this seems to be somewhat negotiable within Trump's own head.

As Philip Bump notes, Candidate Trump regularly alleged that China could reign in North Korea at will. And that continued even after Trump and Xi reportedly got on the same page. Here's what Trump tweeted in March:

By April, Trump suddenly had "great confidence" in Beijing.

By early May, he hailed his “great relationship” with Xi and China, suggesting it was an example of how his foreign policy was getting strong reviews. “I don’t think of it as much, but we’re getting very, very high marks on foreign policy,” Trump told Bloomberg. “Great relationship with China.”

By late last month, though, Trump seemed to basically dismissed China's efforts on the Korean Peninsula, crediting them for trying to deal with North Korea but suggesting he was moving on.

Then last week, during an appearance with South Korea's leader Moon Jae-in at the White House, Trump mentioned that country and Japan as allies in seeking a diplomatic solution on North Korea, notably leaving out China.

Then Monday, after North Korea successfully tested an intercontinental ballistic missile, Trump suggested maybe China's role in helping with North Korea wasn't over after all.

And finally on Wednesday, he seemed to suggest in tweets that China wasn't trying hard enough, claiming that it had increased trade with Kim Jong Un's regime and seeming to threaten to rethink trade policies in the region.

All of these storylines are worth keeping an eye on as Trump forms one of his most significant foreign alliances. And all of them are apparently subject to change at a moment's notice.