Smoke 'em if you got 'em
Indicates that the smoking tobacco is permissible and encouraged.  

Originated in mid-20th century wartime.  Commonly used during small gatherings of allied troops.  Tobacco was a rationed commodity, at the time, and was generally in short supply.

The term was later popularized in Hollywood war movies.  
The commanding officer would typically use the term when opening a meeting in order to get the troops' attention and to relax them (assuming, of course, that they 'had em').   

"It's getting rough out there.  We've got to review strategy. Smoke 'em if you got 'em.  We've got some tough decisions to make...."