Most fentanyl is brought in by Americans and not the cartels themselves. Prohibition of drugs, including alcohol, has always turned into a fiasco that led to a huge growth in the numbers and wealth of the illicit providers. The best way to deal with the drug problems are education and rehabilitation. That doesn't mean "Just say no." That's just saying that you're stupid. It means a clear look at the dangers and how and what to avoid.
As a means to pay for this and to mitigate the single thing that makes drugs the most dangerous, we need to legalize and tax them. The biggest killer is not knowing the dosage or the purity of the drugs you get on the street. Being available in known qualities and quantities would probably cut overdoses by 90% or more. And no one would be getting addicted because they got something they didn't want.