The slogan “trade follows the flag” dates to at least 1862, when the Union tried to discourage trade with the Confederacy. The saying became very popular in the second half of the 19th century and is still used.
 
 
Bartleby.com
E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Trade follows the Flag.
Colonies promote the trade of the mother country. The reference is to the custom of planting the flag of the mother country in every colony.
 
18 June 1862, Grand County Herald (Lancaster, WI), “The Grand Naval Engagement,” pg. 1, col. 6:
Trade follows the flag.
 
22 August 1862, San Francisco (CA) Bulletin, “The Perilous Condition of Southern Ohio and Kentucky,” pg. 3:
He would have no such vanity as “Trade follows the Flag;” buy no cotton or anything else of the people in rebellion, till the war was concluded, and send no gold into the rebel country till he knew to what use they put it.
       
Google Books
My days and nights on the battle-field.
A book for boys.
By Charles Carleton Coffin
Boston, MA: Ticknor and Fields
1864
Pg. 310:
Trade follows the Flag.
 
Google Books
Camp-fire and Cotton-field:
Southern adventure in time of war. Life with the Union armies, and residence on a Louisiana plantation
By Thomas Wallace Knox
New York, NY: Blelock and Company
1865
Pg. 193:
Trade Follows the Flag.
 
OCLC WorldCat record
Assisted colonization : various instances of its successful results, with table illustrating the motto “the trade follows the flag”
Author: C W Eddy
Publisher: London : Edward Stanford, 1870.
Edition/Format:  Book : English
   
OCLC WorldCat record
Trade follows the flag : Mr. Gallinger presented the following article from the Sunset magazine of March 1908, entitled “Trade follows the flag,” by H.A. Evans, naval constructor, United States Navy.
Author: H A Evans; Jacob H Gallinger
Publisher: Washington : [G.P.O., 1908]
Series: Senate document (United States. Congress. Senate), 60th Congress, 1st session, no. 375. 
Edition/Format:  Book : National government publication : English