President Barack Obama's decision on a troop pullout from Afghanistan must be viewed against a changing background of circumstances related to U.S. security.

They are:

1. Personnel shifts among his top security advisers.

2. Acute U.S. budget constraints.

3. Realignment of international forces

4. Changing face of al-Qaida.

Ensuing weeks will see Leon Panetta installed as Defense Secretary, Gen. David Petraeus as CIA director and Gen. Martin Dempsey as chairman of the Joint Chiefs. Hillary Clinton says she will leave as Secretary of State next year. If Barack Obama wins a second presidential term, Sen. John Kerry may well succeed her in that post. If a Republican contender ousts Obama, all four of these positions could be up for grabs. Changing personnel certainly means changes in nuance, and perhaps direction.

The president's decision to withdraw 33,OOO U.S. troops from Afghanistan by September of next year, and 68,000 by the end of 2014, along with troop withdrawals from Afghanistan by allied nations, means that effectively Afghanistan's stability will then largely depend on its own resources.

The withdrawal of those combat forces will mean substantial savings for the U.S. at a time when many Americans at home are facing economic hardships.

Outgoing Defense Secretary Robert Gates has already offered up some cuts in the Pentagon's budget but warned against more. Despite this, the Defense Department can expect some, especially against the background of President Obama's expressed intention to limit any new foreign military involvement.

Then there is the changing lineup and potential of America's potential enemies. The days of the Cold War are long since over.

The two, possibly three, most dangerous nuclear powers for the U.S. today are Iran, Pakistan, and possibly North Korea.

Iran is unpredictable and wily, almost certainly lying about its commitment to the use of nuclear power only for peaceful purposes. An Iranian nuclear strike against Israel, or the delivery of an Iranian bomb to Islamist terrorists for use against the U.S., is a potential any incumbent in the White House must surely contemplate daily.

Pakistan's situation is different. It has an arsenal of nuclear weapons but its government and military assure the U.S. that they are safely secure. Terrorist penetration of military bases and ranks makes such assurances less convincing in Washington. Over the years, rule in Pakistan has veered between civilian and military, and its stability has been in question. With a pathological fear of India, it has balanced its alliances between the U.S. and China. Internally it has hedged its bets between pursuit of some terrorist factions and cooperation with others. It is concerned that whoever comes out on top in Afghanistan not plunge that country on its western border into tumult.

The U.S. shares that concern, but essentially because it wants to forestall chaos in Afghanistan that could destabilize Pakistan and cause its nuclear arsenal to fall into Islamist extremist hands.

Meanwhile North Korea is an unpredictable wildcard, but hopefully understands that delivery of a nuclear weapon to a terrorist group for use against the U.S. would probably entail its own obliteration.

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Finally, al-Qaeda after Osama bin Laden has an impaired leadership but an extended bunch of franchises in Somalia and Yemen and elsewhere seeking American Islamist sleepers to conduct terrorist acts against the American homeland.

The kind of warfare America must fight in light of new circumstances is changing, but nonetheless dangerous. There will be fewer battlefields involving thousands of soldiers, many more drones and small-force counter-insurgency units, and an enormous contest of wits and intelligence.

John Hughes teaches journalism at Brigham Young University. He is a former editor and chief operating officer of the Deseret News, and a former editor of the Christian Science Monitor, which syndicates this column.