MEXICO CITY (Reuters) - An "insatiable" appetite in the United States for illegal drugs is to blame for much of the violence ripping through Mexico, U.S. Secretary of State Hillary Clinton said on Wednesday. Clinton acknowledged the U.S. role in Mexico's vicious drug war as she arrived in Mexico for a two-day visit where she discussed U.S. plans to ramp up security on the border with President Felipe Calderon. A surge in drug gang killings to 6,300 last year and fears the violence could seep over the border has put Mexico's drug war high on President Barack Obama's agenda, after...