This book is the result of three years of research aimed at exposing how the perceptions of Mexico and the United States changed, at a time when their security was being threatened by drug traffickers, to collaborate openly before the public opinion of the two countries and with a specific collaboration scheme known as the Merida Initiative. The idea of co-responsibility and that it was a shared problem to fight drug traffickers meant that the bitter experiences of the past between the two countries were left behind, allowing them to establish a bilateral closeness to confront drug traffickers in both territories. This work also reflects that the commitments, the form of bilateral support and work was a product of past experience, but also of unprecedented political will.