Diplomacy is the art of saying, “Nice doggie,” until you can find a rock.

Will Rogers (1879–1935) American humorist
(Misattributed)

Actually said by Wynn Catlin.

For more information on the background of this quotation, see Quote Origin: Diplomacy Frequently Consists in Soothingly Saying “Nice Doggie” Until You Have a Chance to Pick Up a Rock – Quote Investigator®.