the.com/bone of contention

means The specific point of dispute at the center of an ongoing disagreement — the one issue people keep fighting about.

from Drawn straight from the image of two dogs snarling over a single bone, neither willing to let go. The phrase surfaces in English by the 18th century, building on older 'bone' idioms like 'bone to pick,' which already used the dog-and-bone fight as a stand-in for human quarrels.