Sowell stumbled into something he probably didn't plan to with his analogy. While on the surface, Dunkirk was a 'battle they coldn't win' and a loss, it was actually a very important strategic battle.
The battle of Dunkirk happened after the battle for France had already lost, however, the French uniting to help evacuate the allied soldiers actually created the roots of a major underground resistance in France. At the time, the allies didn't view Dunkirk as a loss but as a victory as those 300K troops were successfully rescued.
Oh, and when all those soldiers were evacuated, they left behind a lot of equipment that was used later by the French resistance.
There is an old book Sowell should read: "The Importance of Dunkirk Consider'd: In Defence of the Guardian of August the 7th" by Sir Richard Steele.
So in this case, Cruz should accept the comparison, because, while the uninformed may look at what he is doing as a 'battle he cannot win', the reality is he is creating the roots of resistance for a larger war.