President Obama made a surprise decision on Tuesday to send 1,200 additional National Guard troops to the border with Mexico and seek more money to combat drug smuggling. It followed a testy meeting with Republican senators, many of whom have been pressing the administration to take drastic action to lock down the border.

Take your pick of possible explanations for Mr. Obama's sudden move: a political tactic to gain cover for immigration reform; a re-election gift to Senator John McCain of Arizona, who is in a desperate primary struggle with a border hawk; a realistic effort to stem illegal flows of drugs and people north, guns and money south (the least-convincing explanation).

Mexico is in a vicious fight to the death with the drug cartels. Thousands of people south of the border have already been killed. While the war has not yet spilled into this country - the violent crime rate is down in many American border cities and towns - anxiety is high. But adding a thousand or so border troops won't stop the cartels or repeal the law of supply and demand. It won't lessen American addicts' hunger for drugs or Mexican traffickers' appetite for sold-in-America guns.

The Obama administration is making other sensible efforts to stem the crime flows: checking southbound rail cars at the border for illegal guns, money and other contraband; helping Mexico battle the cartels; sharing more information among American law-enforcement agencies.

Getting serious about security, though, means looking beyond the border. Addressing Congress last week, President Felipe Calderón of Mexico practically begged lawmakers to reinstate the assault-weapons ban that Washington let expire in 2004, greatly contributing to violence in his country.

Congress also urgently needs to crack down on rogue gun dealers whose sales to straw purchasers help arm the drug cartels and close the notorious loophole that allows gun traffickers to purchase large numbers of weapons from unregulated private sellers at gun shows for shipment across the border. Mr. Obama has avoided these fights - and get-tough-on-the-border Republicans are pressing in the opposite direction.

The Republicans also insist that border security has to come before immigration reform. But a tighter border without a legal path for unskilled labor only makes human smuggling part of the criminals' business plan. Unlike bricks of cocaine, people can be smuggled more than once and their families shaken down for easy cash.

With comprehensive immigration reform, law-abiding workers will come and go with visas, and the people left to illegally cross the Arizona desert will be drug traffickers, gun runners and other undesirables. Then a thousand or so troops might actually make a difference.