You would have to make it legal everywhere or at least as many places as possible. The world followed the US lead on the drug war Im sure they could follow our lead with this too. Especially if you could demonstrate how it would hurt these criminal operations. If prostitution is legalized in 70% of countries, you just destroyed 70% of their human trafficking operations. Yes there would be the remaining 30% of countries but they already are experiencing that the demand isnt going to go up 70% in those few remaining 30% of countries.