What did Donald Trump do today?

He invited China to dictate US policy towards North Korea--and China obliged.

Trump's only public response to news that North Korea has successfully tested an intercontinental ballistic missile came, naturally, via Twitter--the same place he once promised that such a missile launch "won't happen." He suggested that China would "put a heavy move on North Korea and end this nonsense once and for all." He also tweeted a mild rebuke to Kim Jong-un ("Does this guy have anything better to do with his life?"), although given Trump's known fondness for authoritarians, it doesn't seem his recently expressed admiration for Kim has dimmed very much.

Like the United States and other interested nations, China cannot really "put a heavy move" on North Korea without risking a major conflict--or possibly even a nuclear war. But China was quick to take advantage of the leadership role Trump was conceding, and in a joint statement with Russia promptly called for the United States and its ally South Korea to cease conducting naval maneuvers in the region. The statement implicitly blamed the United States' military presence in the region for provoking North Korea.

Both Russia and China compete with the United States for naval superiority in the Sea of Japan and the waters off China's coast. 

Why is this a problem?

• It's bad if the President of the United States asks non-aligned or hostile nations to dictate how nuclear threats are handled.

• Simplistic understanding of complex situations tends to yield stupid policy.

• "Modern day presidents" may conduct nuclear diplomacy via Twitter, but competent presidents don't.