Quoting from the Warning:

"The U.S. Department of State warns U.S. citizens about the risk of traveling to certain parts of Mexico due to the activities of criminal organizations in those areas. U.S. citizens have been the victims of violent crimes, including homicide, kidnapping, carjacking, and robbery in various Mexican states."

The long list of Mexican regions that Americans are cautioned to avoid includes popular tourist areas like Baja California, Acapulco, Puerto Vallarta, Cancun and Cozumel.

In 2014 (most recent data available) the USA was Mexico's #1 tourism source market, with more than 7M visits.

Canada was a distant #2 at 1.7M visitors. Per Canadian federal agency Global Affairs Canada:

"There is no nationwide advisory in effect for Mexico. However, you should exercise a high degree of caution due to high levels of criminal activity, as well as demonstrations, protests and occasional illegal roadblocks throughout the country."

The Canadian government also recommends avoiding non-essential travel to ands in the Northern and Western states of Mexico- essentially the same areas referenced in the US warning.

These alerts create a potentially big problem for Mexican tourism.