PRESIDENT OBAMA failed to offer a convincing military or strategic rationale for the troop withdrawals from Afghanistan that he announced Wednesday night. In several ways, they are at odds with the strategy adopted by NATO, which aims to turn over the war to the Afghan army by the end of 2014. For that plan to succeed, military commanders believe that U.S. and allied forces must hold the areas in southern Afghanistan that have been cleared of the Taliban through this summer’s fighting season as well as that of 2012. They also must sweep eastern provinces that have not yet been reached by the counterinsurgency campaign.

By withdrawing 5,000 U.S. troops this summer and another 5,000 by the end of the year, Mr. Obama will make those tasks harder. By setting September 2012 as a deadline for withdrawing all of the 33,000 reinforcements he ordered in late 2009, the president risks undermining not only the war on the ground but also the effort to draw elements of the Taliban into a political settlement; the militants may prefer to wait out a retreating enemy. It also may be harder to gain cooperation from Pakistan, whose willingness to break with the Taliban is linked to its perception of U.S. determination to remain engaged in the region. U.S. allies, which have committed 40,000 troops to the 2014 plan, may revise their own exit strategies.

An accelerated withdrawal of American forces would make more sense if Mr. Obama had decided to abandon the modified counterinsurgency plan he adopted at the end of 2009, which was later expanded and endorsed by NATO. Vice President Biden, among others, has pressed for a more limited counterterrorism strategy focused on combating al-Qaeda. But Mr. Obama offered no indication in Wednesday’s speech that he has altered his objectives. Instead, he argued that the reduction is possible because “we are achieving our goals . . . . We are starting this drawdown from a position of strength.”

Mr. Obama correctly pointed out that the killing of Osama bin Laden and operations in Pakistan have weakened al-Qaeda and limited its ability to attack the United States. But a Taliban resurgence in Afghanistan, which Mr. Obama’s withdrawals risk, would be deeply destabilizing for a region that includes nuclear-armed Pakistan and India. If the Afghan government or army crumbles, there would be a considerable chance that the United States would lose the bases it now uses for drone attacks against al-Qaeda.

Perhaps the best justification for Mr. Obama’s decision is U.S. domestic opinion. As senior administration officials have pointed out, Americans have grown weary of the war; polls show that a majority support a rapid withdrawal of U.S. forces, and that view is increasingly reflected in Congress and even among Republican presidential candidates. Many in Congress cite the cost of the war — though the few billion dollars saved through a faster withdrawal will have little impact on a deficit measured in trillions.

By announcing these pullouts, Mr. Obama may ease some of the political pressure while still allowing his commanders enough forces to complete the 2014 transition plan. The president’s supporters point out that at the end of 2012, there will still be twice as many U.S. troops in Afghanistan — 68,000 — as when Mr. Obama took office. We hope those prove sufficient. But Mr. Obama’s withdrawal decision, with no clear basis in strategy, increases the risk of failure.