… is from paragraph 5 of Eli Heckscher’s “Foreword” to his 1922 The Continental System: An Economic Interpretation  (Harald Westergaard, ed.):
Nowadays, as in the days of mercantilism, most states, guided by the economic perceptions of the average man, labour in time of peace to render difficult the importation of foreign goods, and at the same time to force their own products on the world market, (although in reality this is incompatible with the former aim). In time of war, however, they suddenly swerve around, either to the inverted standpoint of encouraging imports and hampering exports, or, in general terms, of preventing all trade with the enemy. This statement does not, of course, imply any judgment as to which policy has the greater justification; it is merely an assertion of the at least seemingly greater inconsistency of our present procedure.
It’s an enduring mystery – one stirred up by the swirling torrent of errors and illogic that is mercantilism – that so many people (many of whom seem otherwise to be intelligent) will assert, with one breath, “That country is our enemy. Let’s make it poorer by blocking its citizens’ abilities to trade with foreigners.” and then with the next breath exclaim “Our country needs economic help. Let’s make it richer by blocking our citizens’ abilities to trade with foreigners.”