With our record of military losses around the world do you want to risk losing a war to the cartels on the border - and I am not being sarcastic. I am being sardonic.
Political interference. Using the Schwarzkopf policy of winning wars, the US would win the war.
However, corrupt politicians would other wise interfere and then you have possible terrorist attacks within the country because the cartels all own the drug distribution routes in America and they get a cut from everybody that is involved in the drug trade.