It emboldens the Taliban, which thanks to Mr. Obama’s surge and David Petraeus’s generalship had all but been ousted from its traditional strongholds in Kandahar and Helmand provinces. “My soul, and the soul of thousands of Taliban who have been blown up, are happy,” Taliban field commander Jamal Khan told the Daily Beast of his reaction to Mr. Obama’s speech. “I had more than 50 encounters with U.S. forces and their technology. But the biggest difference in ending this war was not technology but the more powerful Islamic ideology and religion.”

It increases the risk to U.S. forces in Afghanistan, where the fatality count was finally starting to come down after peaking in 2010. Fewer troops means that U.S. commanders will have to make an invidious choice between clearing territory of enemies and holding and building it for friends. “Whether it is Nangarhar or Ghazni, Kandahar or Herat, the place where we decide to ‘surge’ with remaining forces will leave a window open—and the Taliban will crawl in,” says a U.S. military official with experience in Afghanistan. “Any commander who has experienced a withdrawal under pressure knows that it is perhaps the most difficult operation you can conduct and certainly the most dangerous; it gives the attacker a feeling of superiority and demoralizes the withdrawing force.”

It strengthens already potent anti-American forces in Pakistan and weakens the hand of moderates. Skeptics of the U.S. within Pakistan’s government, particularly the army, will mark the U.S. withdrawal as further evidence that Washington is a congenitally unreliable ally. Opponents of the U.S. outside of the government will capitalize politically on the perception of American weakness. Drone strikes, which outgoing CIA director Leon Panetta has called “the only game in town in terms of confronting or trying to disrupt the al Qaeda leadership,” will likely come to an end. Islamabad will also find new reasons to patch up its differences with erstwhile allies in the Taliban and other terrorist groups as a way of keeping its options open.

It strengthens the hand of Iran, which, as the Journal’s Jay Solomon reported yesterday, “is moving to cement ties with the leaders of three key American allies—Afghanistan, Pakistan and Iraq—highlighting Tehran’s efforts to take a greater role in the region as the U.S. military pulls out.” As a demonstration of those ties, Iranian Defense Minister Ahmad Vahidi paid a visit to Kabul last week to sign a bilateral security agreement. “We believe that expansion of joint defense and security cooperation with Iran is in favor of our interests,” said his Afghan counterpart Abdulrahim Wardak.

It further weakens NATO, whose future is already in doubt given its inability to oust Moammar Gadhafi from Tripoli. In the last decade it became the fashion to say of the alliance that it was either “out of area”—meaning Europe—or “out of business.” Leaving and losing Afghanistan spells the latter.

It gives Hamid Karzai opportunity and motive to reinvent himself as an anti-American leader. The Afghan president is already well on his way to forging a close political alliance with insurgent leader Gulbuddin Hekmatyar, who is believed to have given Osama bin Laden safe passage out of Afghanistan in 2001 and is wanted by the U.S. on a $25 million bounty. Mr. Karzai is said to be furious that the Obama administration made no effort to get a strategic forces agreement that would have left a residual U.S. force after 2014. “I think the reality of their complete withdrawal has struck home,” Afghan Human Rights Commissioner Nader Nadery tells the Associated Press. “Now he sees they may go and they don’t want a [military] presence here . . . and perhaps now he is thinking, ‘Who will protect me?'”

It accelerates Afghanistan’s barely suppressed, and invariably violent, centrifugal forces. There are already reports that the old Northern Alliance, which held out against the Taliban in the 1990s and took Kabul in 2001, may be reconstituting itself as a fighting force in anticipation of a hostile government in Kabul. This is a formula for civil, and perhaps regional, war; it is not clear what kind of “partnership” the U.S. could hope to build, as Mr. Obama promised to do, with whatever emerges from its ashes.

Finally, it signals that the United States, like Britain before it, is a waning power. In his speech last week, Mr. Obama waxed eloquent on the point that “what sets America apart is not solely our power—it is the principles upon which our union was founded.” Very true. But a nation that abandons to the Taliban those it was once committed to protect shows that it lacks power and principle alike.

At the end of “Charlie Wilson’s War,” the film quotes the late congressman saying: “These things happened. They were glorious and they changed the world. . . . And then we f—ed up the endgame.” To watch President Obama’s Afghan policy unfold is to understand exactly what Wilson meant.

Write to bstephens@wsj.com