Why does Bern advise against fighting Pug to free Caspian’s friends?

Bern tells Caspian that fighting Pug to free his friends would be a war he could not win... that as soon as Caspian attempted to fight.... additional ships would appear to fight against him. Bern suggests he change his plans and concentrate on making himself look more powerful than he is..... and also using his position as king to awe his people.