“This is no idle danger,” Mr. Obama said, “no hypothetical threat.” He warned that new attacks were being plotted in the region, and raised the terrifying prospect of an unchecked Al Qaeda taking over a nuclear-armed Pakistan.

Mr. Obama’s decision to send an additional 30,000 troops  and ask NATO allies for several thousand more  is unlikely to end the political debate. Republicans are certain to point out that it is still short of the 40,000 requested by the top field commander, Gen. Stanley McChrystal, and object to the president’s pledge of a quick drawdown. Many Democrats and the president’s own vice president had opposed any escalation.

At this late date, we don’t know if even 100,000 American troops plus 40,000 from NATO will be enough to turn the war around. But we are sure that continuing President Bush’s strategy of fighting on the cheap (in January 2008, the start of Mr. Bush’s last year in office and more than six years after the war began, there were only 27,000 American troops in Afghanistan) is a guarantee of defeat.

Mr. Obama said he planned to move those 30,000 troops in quickly  within six months  to break the Taliban’s momentum, secure key population centers, speed up training of Afghan security forces and then hand over control to Afghan authorities. He said he expected to be able to start drawing down American forces in July 2011. But he made no promise about when all American combat troops would be gone, saying only that the decision would be based on conditions on the ground.

Over all, we found the president’s military arguments persuasive.

The Afghan people have no love for the Taliban’s medieval ideas and brutality, but the Karzai government’s failure to provide basic services or security has led many to conclude that they have no choice but to submit. Driving the Taliban back swiftly and decisively from key cities and regions should help change that calculation. Coupled with an offer of negotiations, it may also peel away less committed fighters.