MEXICO CITY, Oct. 22 -- President Bush announced Monday in Washington that he will ask Congress to approve a $500 million package to help Mexico fight drug cartels, the largest international anti-drug effort by the United States in nearly a decade.

The proposal could represent a seismic shift in relations between the two countries, whose law enforcement agencies and policymakers have often bickered over the drug war, as well as other hugely contentious issues such as immigration reform and trade.

U.S. and Mexican negotiators reached the agreement in secrecy. Some in Mexico worried that an aid package would infringe upon its sovereignty, and concerns surfaced in the United States about costs and strategy in light of the oft-criticized effort to combat drugs in Colombia.

The much-anticipated Mexico aid plan, which is included in the president's $46 billion supplemental budget request for war funding, would pay for helicopters, canine units, communications gear and inspection equipment, the State Department said.

The program also would include training and technical advice on vetting new police officers, and case-management software to track investigations in a nation where drug kingpins have infiltrated many state and local governments and infighting among drug traffickers has cost more than 4,000 lives in the past 22 months.

The violence is particularly acute in northern Mexico, where gunfights frequently spill across the U.S. border, a major reason congressional delegations in Texas and other border states have pushed for the aid deal.

Mexico's drug cartels have been engaged in a fierce war for at least two years as they compete for lucrative trade routes and to try to fill power vacuums left after the extradition of several major cartel leaders to face trial in the United States.

Although the bulk of U.S. attention is focused on Mexico, Bush also announced an additional $50 million in proposed aid for Central American nations that have been beset by rampant violence and drug cartel corruption as traffickers seek new routes for the tons of cocaine and other drugs that flow into the United States every day.

The aid packages are part of what the Bush administration hopes will be a multiyear, $1.4 billion initiative.

Bush barely mentioned the package in his budget remarks. But within minutes of his announcement, the White House -- cognizant of possible opposition in Congress -- launched a public relations offensive, distributing a statement about the aid plan that was followed by enthusiastic news releases from the U.S. ambassador to Mexico, Antonio O. Garza Jr., and the State Department.

"This initiative . . . represents a fundamental shift in strengthening our strategic partnership and is the single most aggressive undertaking ever to combat Mexican drug cartels," Garza said.

In a conference call with reporters, Thomas A. Shannon Jr., the State Department's top diplomat for the Western Hemisphere, hailed the president's request as "historic" and predicted it could create "a new paradigm" in U.S.-Mexico relations.