In U.S. President Donald Trump's calculus, a choice between Chinese cooperation or American military action loom large as part of any solution to the North Korean nuclear crisis. But one often unspoken aspect of this outlook has been Beijing's rarely mentioned mutual defense pact with Pyongyang — a treaty that would oblige China to defend North Korea in event of an attack.

The little-discussed pact, inked in 1961, legally requires Beijing to "immediately render military and other assistance by all means at its disposal" in the event the North is attacked. Such assistance could simply mean providing better defensive weapons, but it could also include something dramatic, like deploying troops and conducting military actions against attacking countries.

For both countries, this alliance — sealed in blood by the Korean War — remains relevant and personal.