One definition of a diplomat comes from the Cold War:
 
“A diplomat is a person who can be disarming even though his country isn’t.”
 
The Saturday Evening Post of February 16, 1956 credited the line—without any further explanation—to Sydney Brody.
 
     
Google Books
18 February 1956, The Saturday Evening Post, pg. 91, col. 2:
A diplomat is a person who can be disarming even though his country isn’t.
SYDNEY BRODY
 
Google Books
December 1956, Boys’ Life, “Think and Grin,” pg. 98, col. 3:
Daffynishion : Diplomat — A person who can be disarming even if his country isn’t. — Tim Londergan, Niagara Falls, N. Y.
   
Google Books
Handbook of Humor for All Occasions
By Jacob Morton Braude
London: Bailey Bros. & Swinfen
1958
Pg. 276
Dilplomat:
(...)
7. a person who can be disarming even though his country isn’t.
 
1 April 1959, Chicago (IL) Daily Tribune, ‘Tower Ticker” by Herb Lyon, pt. 3, pg. 2, col. 5:
Jim [Cafe Bohemia] Janek defines a Russian diplomat as A fellow who can be disarming even tho his country isn’t!”
 
31 May 1960, Boston (MA) Evening American, “New Mother Goes to WOrk as Dad Relaxes” by Hal Boyle, pg. 40, col. 3:
Thoughtful definition: “A diplomat,” says the Catholic Digest, “is a fellow who can be disarming, even though his country isn’t.”
   
Google Books
The Modern Handbook of Humor
By Ralph Louis Woods
New York, NY: McGraw-Hill
1967    
Pg. 142:
Diplomat: A person who can be disarming even though his country isn’t. — Sydney Brody
 
Google Books
Roast of the Town
By Joey Adams
New York, NY: Prentice Hall
1986
Pg. 169:
A diplomat is a person who can be disarming even though his country isn’t.
 
Twitter
Steven Dolby
‏@StevenDolby
“A diplomat is a person who can be disarming, even if his country isn’t.” Sidney Brody
4:06 PM - 28 Jul 2013
 
Twitter
VX International
‏@pmhildreth
Diplomat: a person who can be disarming even though his country isn’t - Charles G Dawes #quotes #diplomacy #politics
3:53 AM - 12 Jan 2015