draw a line in the sand

Short definition from Wiktionary

Verb

1. 1
   To lay down a challenge; to provide a test of commitment (often one which carries a high risk) to a cause.
2. 2
   To create a real or artificial boundary or distinction between (two places, people or things).
3. 3
   To indicate the threshold or level above which something will become unacceptable or will provoke a response; to create a boundary and imply or declare that its crossing will provoke a (negative) response.