Usually an enemy does not appear uncovered and plainly; and the foe under the guise of friend is difficult to be knownِhow can a bird know that the branch upon which he sings sweetly, will one day become a cage for him!?
The brothers too, concealed their plot under the cover of brotherly affection, so that neither Jacob nor Joseph could openly resist against their request.
This is what we see to-day, that our enemies come to us in the beautiful forms of economical support, social and cultural help, military advisors, and etc. We sometimes see that the most dangerous spies of our enemy come to our house or land, under the guise of Embassador or supporters of human rights!!