CIUDAD JUAREZ, Mexico - Drug cartels are sending a brutal message to police and soldiers in cities across Mexico: Join us or die.

The threat appears in recruiting banners hung across roadsides and in publicly posted death lists. Cops get warnings over their two-way radios. At least four high-ranking police officials were gunned down this month, including Mexico's acting federal police chief.

Mexico has battled for years to clean up its security forces and win them the public's respect. But Mexicans generally assume police and even soldiers are corrupt until proven otherwise, and the honest ones lack resources, training and the assurance that their colleagues are watching their backs.

Police who take on cartels feel isolated and vulnerable when they become targets, as did 22 commanders in Ciudad Juarez when drug traffickers named them on a handwritten death list left at a monument to fallen police.

Of the 22, seven have been killed and three wounded in assassination attempts. Of the others, all but one have quit, and city officials said he didn't want to be interviewed.

Yesterday, city spokesman Sergio Belmonte confirmed that Juarez's police chief had submitted his resignation and said he would be replaced by a military official on leave from the armed forces.

"These are attacks directed at the top commanders of the city police, and it is not just happening in Ciudad Juarez," Mayor Jose Reyes Ferriz said at the funeral of the latest victim, police director Juan Antonio Roman Garcia.

The killings are in response to a crackdown launched by President Felipe Calderon, who has sent thousands of soldiers and federal police across the nation to confront the cartels. Drug lords have hit back by sending killers to attack police with hand grenades and assault rifles.

Police are increasingly giving up. Last week, U.S. officials revealed that three Mexican police commanders have crossed into the U.S. to request asylum, saying they fear for their lives. *