In Korea, weapons were limited by the qualitative distinction between atomic and all other; it would surely have been much more difficult to stabilize a tacit acceptance of any limit on size of atomic weapons or selection of targets. {37} No definition of size or target is so obvious and natural that it goes without saying, except for “no size, on any target.” American assistance to the French forces in Indochina was persuasively limited to material, not people; and it was appreciated that an enlargement to include, say, air participation could be recognized as limited to air, while it would not be possible to establish a limited amount of air or ground participation. One’s intentions to abstain from ground intervention can be conveyed by the complete withholding of ground forces; one cannot nearly so easily commit some forces and communicate a persuasive limit to the amount that one intends to commit.

QUESTION: If no natural qualitative threshold exists in a domain, how do tacit limits ever form there?

Thomas C. Schelling, The Strategy of Conflict, loc. 194