President Bush and Democratic contender John Kerry have offered certain advice on U.S. troop positioning that each would do well to follow.

In a period of rapid change, uncertainty, turmoil and terrorism threats, this nation should send a signal of support to the world, not one of premature withdrawal.

The issue of troop redeployment has emerged as one of the few clear strategic differences between Bush and Kerry.

The Bush administration, citing longstanding Pentagon and State Department efforts, would call home some 70,000 U.S. troops from posts in Europe and Asia. Kerry has argued to leave those troops in place. However, he has suggested that he would endeavor to begin withdrawing American forces from Iraq within a half-year of assuming the presidency.

If the United States were dealing with the post-Cold War world that appeared to take shape in the 1990s, one of expanding economic prosperity and peace dividends, and if the war in Iraq showed evidence of success, Bush’s and Kerry’s recommendations would make sense. But those are not modern realities.

Europe, while moving rapidly toward enhanced integration, has hosted — in the Balkans — one of the world’s most disruptive regional wars of the past decade.

During the same period, in North Korea, the United States came disturbingly close to a conflict that could have involved nuclear weapons for only the third time since World War II.

Then, on Sept. 11, 2001, terrorists added immeasurably to the world’s troubles, heralding a long struggle that would require a creative, continuous and cooperative response. Subsequent terrorist attacks, notably in Madrid, have underlined the threat.

So why engage in a troop redeployment from Europe that could strain alliances? It would be difficult, as one example, for the United States to escape the perception that it wishes to punish Germany — which stands to lose tens of thousands of American troops — for its lack of support in Iraq. Beyond that consideration, the United States could well need its forces in Europe for future missions against security threats there and in surrounding regions.

In North Korea, the challenge differs. Pyongyang’s nuclear-weapons ambitions, which negotiations appeared to tame a decade ago, have resurfaced in recent years. The North Koreans could interpret a partial U.S. troop withdrawal from South Korea as Washington’s bowing to their nuclear saber-rattling. Or, even worse, given the North Koreans’ paranoia, they could view a withdrawal as a pre-invasion ritual to safeguard U.S. troops and launch a preemptive strike.

Some critics say that one motive behind Bush’s redeployment plan is to secure more troops for Iraq. Well, the Iraqi conundrum requires a larger military force. If other countries fail to deliver, though, the burden will fall to America.

But under any likely circumstances, the United States will not find itself in a position to bring troops home from Iraq by this time next year — unless Washington shirks its duty and abandons Baghdad to chaos and terrorism.

Until a safer world emerges, U.S. troops have an obligation to stand firm on all fronts.

John C. Bersia, who won a Pulitzer Prize in editorial writing for the Orlando Sentinel in 2000, is also the special assistant to the president for global perspectives and a professor at the University of Central Florida. .