The secretive North Korean regime has always been known for bluster and hyperbole, but in recent days its pronouncements have become more extreme, and the reasons, as usual, are somewhat obscure.

Pyongyang could be reacting to United Nations sanctions approved last week or ongoing military exercises between the United States and South Korea, which always rile up the excitable northerners. But the real trigger may have been China’s decision to crack down, at last, on its erratic ally.

North Korea in recent months has defied the world — and its Chinese allies — by testing an intercontinental ballistic missile and conducting a third nuclear bomb test. Beijing fears the North’s reckless pursuit of nuclear bombs could set off an East Asian arms race that would undermine its own regional security.

North Korea says it has torn up the armistice of 1953 ending the Korean War — although no official notification had been yet sent to Washington by late Monday — and is refusing to answer calls from South Korea on their shared hot line.

The Ask a Korean blog, which tracks Korean-language media, reports a new sense of resolve on the part of the Beijing leadership. It has imposed tougher enforcement of smuggling across China’s border with North Korea, causing prices for essential commodities like rice to skyrocket in North Korea.

China’s move to crimp off cross-border trade could have prompted Pyongyang to revert to its default mode: Issue a stream of dire threats to gain attention, leading to another round of talks and more shipments of aid in exchange for Pyongyang’s promise, once again, to cease and desist.

That’s how it has worked for the last 15 years. The tougher response by the United Nations as well as the Obama administration — which announced tough sanctions of its own Monday — indicates that the old game of Pyongyang blackmail is rightly no longer working.