MEXICAN PRESIDENT Felipe Calderon arrived in Washington on Thursday deeply frustrated with the United States - and with good reason. For four years, the courageous Mexican leader has been waging war against drug cartels that threaten to destroy the rule of law in Mexico at a cost of thousands of lives and tens of billions of dollars. Success has been elusive - in large part because the traffickers continue to receive a steady supply of weapons and cash from the United States. While promising partnership to Mr. Calderon, Congress and the Obama administration have failed to deliver.

In a meeting with editors and reporters of The Post, Mr. Calderon spelled out some of those failures. In four years, Mexico has confiscated 110,000 weapons, including more than 50,000 assault rifles, 11,000 grenades and more than 150 high-powered sniper rifles, as well as 10 million rounds of ammunition. Eighty-five percent of this vast arsenal has been smuggled in from the United States - thanks to Congress's refusal to reinstate a ban on assault weapons and the failure to adequately police the thousands of gun merchants in border states. One of those smuggled weapons, obtained at a gun show in Texas, was used in the killing of a U.S. special agent in Mexico last month.

Meanwhile, U.S. demand for drugs is growing, because of what Mr. Calderon politely described as "some contradictions in public policies." Pointing to California's legalization of medical marijuana, he said that Mexico could accept that "American authorities will enforce the law - or that American authorities will take the courage to legalize. But we cannot live with this contradiction."

Then there is the aid to Mexico first proposed by President George W. Bush - $1.3 billion for training and equipment, including patrol boats and Black Hawk helicopters. The money was to be delivered in three years; in four, Mexico has received $500 million. Mr. Calderon understandably wonders about "the cruising speed of the American bureaucracy" and asks, "how urgent is that?"

Mr. Calderon is not shifting blame. He frankly acknowledges his own government's shortcomings, including a failure to act effectively against money laundering and continuing problems with local police forces, many of which have been penetrated by the traffickers. When he describes the U.S. performance as "clearly insufficient," he is simply telling the truth. "It's not a question of money," he said. "It's a question of co-responsibility. It's a common problem we have to address together."

After meeting the Mexican leader at the White House, President Obama gracefully acknowledged that last point and, like Mr. Bush before him, praised Mr. Calderon for his courage. He said that his administration had stepped up prosecutions of gun runners. Yet Mr. Obama has not pressed Congress about assault weapons and under pressure has retreated from modest measures to tighten regulation of gun shops. It would take much less courage than Mr. Calderon has shown for the American president to take on the forces that impede the fight against drug traffickers. Mexico's leader has a right to be disappointed.